@c -*-texinfo-*- @c This is part of the GNU Emacs Lisp Reference Manual. @c Copyright (C) 1990-1995, 1998-1999, 2001-2019 Free Software @c Foundation, Inc. @c See the file elisp.texi for copying conditions. @node Searching and Matching @chapter Searching and Matching @cindex searching GNU Emacs provides two ways to search through a buffer for specified text: exact string searches and regular expression searches. After a regular expression search, you can examine the @dfn{match data} to determine which text matched the whole regular expression or various portions of it. @menu * String Search:: Search for an exact match. * Searching and Case:: Case-independent or case-significant searching. * Regular Expressions:: Describing classes of strings. * Regexp Search:: Searching for a match for a regexp. * POSIX Regexps:: Searching POSIX-style for the longest match. * Match Data:: Finding out which part of the text matched, after a string or regexp search. * Search and Replace:: Commands that loop, searching and replacing. * Standard Regexps:: Useful regexps for finding sentences, pages,... @end menu The @samp{skip-chars@dots{}} functions also perform a kind of searching. @xref{Skipping Characters}. To search for changes in character properties, see @ref{Property Search}. @node String Search @section Searching for Strings @cindex string search These are the primitive functions for searching through the text in a buffer. They are meant for use in programs, but you may call them interactively. If you do so, they prompt for the search string; the arguments @var{limit} and @var{noerror} are @code{nil}, and @var{repeat} is 1. For more details on interactive searching, @pxref{Search,, Searching and Replacement, emacs, The GNU Emacs Manual}. These search functions convert the search string to multibyte if the buffer is multibyte; they convert the search string to unibyte if the buffer is unibyte. @xref{Text Representations}. @deffn Command search-forward string &optional limit noerror count This function searches forward from point for an exact match for @var{string}. If successful, it sets point to the end of the occurrence found, and returns the new value of point. If no match is found, the value and side effects depend on @var{noerror} (see below). In the following example, point is initially at the beginning of the line. Then @code{(search-forward "fox")} moves point after the last letter of @samp{fox}: @example @group ---------- Buffer: foo ---------- @point{}The quick brown fox jumped over the lazy dog. ---------- Buffer: foo ---------- @end group @group (search-forward "fox") @result{} 20 ---------- Buffer: foo ---------- The quick brown fox@point{} jumped over the lazy dog. ---------- Buffer: foo ---------- @end group @end example The argument @var{limit} specifies the bound to the search, and should be a position in the current buffer. No match extending after that position is accepted. If @var{limit} is omitted or @code{nil}, it defaults to the end of the accessible portion of the buffer. @kindex search-failed What happens when the search fails depends on the value of @var{noerror}. If @var{noerror} is @code{nil}, a @code{search-failed} error is signaled. If @var{noerror} is @code{t}, @code{search-forward} returns @code{nil} and does nothing. If @var{noerror} is neither @code{nil} nor @code{t}, then @code{search-forward} moves point to the upper bound and returns @code{nil}. @c I see no prospect of this ever changing, and frankly the current @c behavior seems better, so there seems no need to mention this. @ignore (It would be more consistent now to return the new position of point in that case, but some existing programs may depend on a value of @code{nil}.) @end ignore The argument @var{noerror} only affects valid searches which fail to find a match. Invalid arguments cause errors regardless of @var{noerror}. If @var{count} is a positive number @var{n}, the search is done @var{n} times; each successive search starts at the end of the previous match. If all these successive searches succeed, the function call succeeds, moving point and returning its new value. Otherwise the function call fails, with results depending on the value of @var{noerror}, as described above. If @var{count} is a negative number @minus{}@var{n}, the search is done @var{n} times in the opposite (backward) direction. @end deffn @deffn Command search-backward string &optional limit noerror count This function searches backward from point for @var{string}. It is like @code{search-forward}, except that it searches backwards rather than forwards. Backward searches leave point at the beginning of the match. @end deffn @deffn Command word-search-forward string &optional limit noerror count This function searches forward from point for a word match for @var{string}. If it finds a match, it sets point to the end of the match found, and returns the new value of point. Word matching regards @var{string} as a sequence of words, disregarding punctuation that separates them. It searches the buffer for the same sequence of words. Each word must be distinct in the buffer (searching for the word @samp{ball} does not match the word @samp{balls}), but the details of punctuation and spacing are ignored (searching for @samp{ball boy} does match @samp{ball. Boy!}). In this example, point is initially at the beginning of the buffer; the search leaves it between the @samp{y} and the @samp{!}. @example @group ---------- Buffer: foo ---------- @point{}He said "Please! Find the ball boy!" ---------- Buffer: foo ---------- @end group @group (word-search-forward "Please find the ball, boy.") @result{} 39 ---------- Buffer: foo ---------- He said "Please! Find the ball boy@point{}!" ---------- Buffer: foo ---------- @end group @end example If @var{limit} is non-@code{nil}, it must be a position in the current buffer; it specifies the upper bound to the search. The match found must not extend after that position. If @var{noerror} is @code{nil}, then @code{word-search-forward} signals an error if the search fails. If @var{noerror} is @code{t}, then it returns @code{nil} instead of signaling an error. If @var{noerror} is neither @code{nil} nor @code{t}, it moves point to @var{limit} (or the end of the accessible portion of the buffer) and returns @code{nil}. If @var{count} is a positive number, it specifies how many successive occurrences to search for. Point is positioned at the end of the last match. If @var{count} is a negative number, the search is backward and point is positioned at the beginning of the last match. @findex word-search-regexp Internally, @code{word-search-forward} and related functions use the function @code{word-search-regexp} to convert @var{string} to a regular expression that ignores punctuation. @end deffn @deffn Command word-search-forward-lax string &optional limit noerror count This command is identical to @code{word-search-forward}, except that the beginning or the end of @var{string} need not match a word boundary, unless @var{string} begins or ends in whitespace. For instance, searching for @samp{ball boy} matches @samp{ball boyee}, but does not match @samp{balls boy}. @end deffn @deffn Command word-search-backward string &optional limit noerror count This function searches backward from point for a word match to @var{string}. This function is just like @code{word-search-forward} except that it searches backward and normally leaves point at the beginning of the match. @end deffn @deffn Command word-search-backward-lax string &optional limit noerror count This command is identical to @code{word-search-backward}, except that the beginning or the end of @var{string} need not match a word boundary, unless @var{string} begins or ends in whitespace. @end deffn @node Searching and Case @section Searching and Case @cindex searching and case By default, searches in Emacs ignore the case of the text they are searching through; if you specify searching for @samp{FOO}, then @samp{Foo} or @samp{foo} is also considered a match. This applies to regular expressions, too; thus, @samp{[aB]} would match @samp{a} or @samp{A} or @samp{b} or @samp{B}. If you do not want this feature, set the variable @code{case-fold-search} to @code{nil}. Then all letters must match exactly, including case. This is a buffer-local variable; altering the variable affects only the current buffer. (@xref{Intro to Buffer-Local}.) Alternatively, you may change the default value. In Lisp code, you will more typically use @code{let} to bind @code{case-fold-search} to the desired value. Note that the user-level incremental search feature handles case distinctions differently. When the search string contains only lower case letters, the search ignores case, but when the search string contains one or more upper case letters, the search becomes case-sensitive. But this has nothing to do with the searching functions used in Lisp code. @xref{Incremental Search,,, emacs, The GNU Emacs Manual}. @defopt case-fold-search This buffer-local variable determines whether searches should ignore case. If the variable is @code{nil} they do not ignore case; otherwise (and by default) they do ignore case. @end defopt @defopt case-replace This variable determines whether the higher-level replacement functions should preserve case. If the variable is @code{nil}, that means to use the replacement text verbatim. A non-@code{nil} value means to convert the case of the replacement text according to the text being replaced. This variable is used by passing it as an argument to the function @code{replace-match}. @xref{Replacing Match}. @end defopt @node Regular Expressions @section Regular Expressions @cindex regular expression @cindex regexp A @dfn{regular expression}, or @dfn{regexp} for short, is a pattern that denotes a (possibly infinite) set of strings. Searching for matches for a regexp is a very powerful operation. This section explains how to write regexps; the following section says how to search for them. @findex re-builder @cindex regular expressions, developing For interactive development of regular expressions, you can use the @kbd{M-x re-builder} command. It provides a convenient interface for creating regular expressions, by giving immediate visual feedback in a separate buffer. As you edit the regexp, all its matches in the target buffer are highlighted. Each parenthesized sub-expression of the regexp is shown in a distinct face, which makes it easier to verify even very complex regexps. @menu * Syntax of Regexps:: Rules for writing regular expressions. * Regexp Example:: Illustrates regular expression syntax. @ifnottex * Rx Notation:: An alternative, structured regexp notation. @end ifnottex * Regexp Functions:: Functions for operating on regular expressions. @end menu @node Syntax of Regexps @subsection Syntax of Regular Expressions @cindex regexp syntax @cindex syntax of regular expressions Regular expressions have a syntax in which a few characters are special constructs and the rest are @dfn{ordinary}. An ordinary character is a simple regular expression that matches that character and nothing else. The special characters are @samp{.}, @samp{*}, @samp{+}, @samp{?}, @samp{[}, @samp{^}, @samp{$}, and @samp{\}; no new special characters will be defined in the future. The character @samp{]} is special if it ends a character alternative (see later). The character @samp{-} is special inside a character alternative. A @samp{[:} and balancing @samp{:]} enclose a character class inside a character alternative. Any other character appearing in a regular expression is ordinary, unless a @samp{\} precedes it. For example, @samp{f} is not a special character, so it is ordinary, and therefore @samp{f} is a regular expression that matches the string @samp{f} and no other string. (It does @emph{not} match the string @samp{fg}, but it does match a @emph{part} of that string.) Likewise, @samp{o} is a regular expression that matches only @samp{o}. Any two regular expressions @var{a} and @var{b} can be concatenated. The result is a regular expression that matches a string if @var{a} matches some amount of the beginning of that string and @var{b} matches the rest of the string. As a simple example, we can concatenate the regular expressions @samp{f} and @samp{o} to get the regular expression @samp{fo}, which matches only the string @samp{fo}. Still trivial. To do something more powerful, you need to use one of the special regular expression constructs. @menu * Regexp Special:: Special characters in regular expressions. * Char Classes:: Character classes used in regular expressions. * Regexp Backslash:: Backslash-sequences in regular expressions. @end menu @node Regexp Special @subsubsection Special Characters in Regular Expressions @cindex regexp, special characters in Here is a list of the characters that are special in a regular expression. @need 800 @table @asis @item @samp{.}@: @r{(Period)} @cindex @samp{.} in regexp is a special character that matches any single character except a newline. Using concatenation, we can make regular expressions like @samp{a.b}, which matches any three-character string that begins with @samp{a} and ends with @samp{b}. @item @samp{*} @cindex @samp{*} in regexp is not a construct by itself; it is a postfix operator that means to match the preceding regular expression repetitively as many times as possible. Thus, @samp{o*} matches any number of @samp{o}s (including no @samp{o}s). @samp{*} always applies to the @emph{smallest} possible preceding expression. Thus, @samp{fo*} has a repeating @samp{o}, not a repeating @samp{fo}. It matches @samp{f}, @samp{fo}, @samp{foo}, and so on. @cindex backtracking and regular expressions The matcher processes a @samp{*} construct by matching, immediately, as many repetitions as can be found. Then it continues with the rest of the pattern. If that fails, backtracking occurs, discarding some of the matches of the @samp{*}-modified construct in the hope that this will make it possible to match the rest of the pattern. For example, in matching @samp{ca*ar} against the string @samp{caaar}, the @samp{a*} first tries to match all three @samp{a}s; but the rest of the pattern is @samp{ar} and there is only @samp{r} left to match, so this try fails. The next alternative is for @samp{a*} to match only two @samp{a}s. With this choice, the rest of the regexp matches successfully. @strong{Warning:} Nested repetition operators can run for an indefinitely long time, if they lead to ambiguous matching. For example, trying to match the regular expression @samp{\(x+y*\)*a} against the string @samp{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz} could take hours before it ultimately fails. Emacs must try each way of grouping the @samp{x}s before concluding that none of them can work. Even worse, @samp{\(x*\)*} can match the null string in infinitely many ways, so it causes an infinite loop. To avoid these problems, check nested repetitions carefully, to make sure that they do not cause combinatorial explosions in backtracking. @item @samp{+} @cindex @samp{+} in regexp is a postfix operator, similar to @samp{*} except that it must match the preceding expression at least once. So, for example, @samp{ca+r} matches the strings @samp{car} and @samp{caaaar} but not the string @samp{cr}, whereas @samp{ca*r} matches all three strings. @item @samp{?} @cindex @samp{?} in regexp is a postfix operator, similar to @samp{*} except that it must match the preceding expression either once or not at all. For example, @samp{ca?r} matches @samp{car} or @samp{cr}; nothing else. @anchor{Non-greedy repetition} @item @samp{*?}, @samp{+?}, @samp{??} @cindex non-greedy repetition characters in regexp These are @dfn{non-greedy} variants of the operators @samp{*}, @samp{+} and @samp{?}. Where those operators match the largest possible substring (consistent with matching the entire containing expression), the non-greedy variants match the smallest possible substring (consistent with matching the entire containing expression). For example, the regular expression @samp{c[ad]*a} when applied to the string @samp{cdaaada} matches the whole string; but the regular expression @samp{c[ad]*?a}, applied to that same string, matches just @samp{cda}. (The smallest possible match here for @samp{[ad]*?} that permits the whole expression to match is @samp{d}.) @item @samp{[ @dots{} ]} @cindex character alternative (in regexp) @cindex @samp{[} in regexp @cindex @samp{]} in regexp is a @dfn{character alternative}, which begins with @samp{[} and is terminated by @samp{]}. In the simplest case, the characters between the two brackets are what this character alternative can match. Thus, @samp{[ad]} matches either one @samp{a} or one @samp{d}, and @samp{[ad]*} matches any string composed of just @samp{a}s and @samp{d}s (including the empty string). It follows that @samp{c[ad]*r} matches @samp{cr}, @samp{car}, @samp{cdr}, @samp{caddaar}, etc. You can also include character ranges in a character alternative, by writing the starting and ending characters with a @samp{-} between them. Thus, @samp{[a-z]} matches any lower-case @acronym{ASCII} letter. Ranges may be intermixed freely with individual characters, as in @samp{[a-z$%.]}, which matches any lower case @acronym{ASCII} letter or @samp{$}, @samp{%} or period. However, the ending character of one range should not be the starting point of another one; for example, @samp{[a-m-z]} should be avoided. A character alternative can also specify named character classes (@pxref{Char Classes}). This is a POSIX feature. For example, @samp{[[:ascii:]]} matches any @acronym{ASCII} character. Using a character class is equivalent to mentioning each of the characters in that class; but the latter is not feasible in practice, since some classes include thousands of different characters. A character class should not appear as the lower or upper bound of a range. The usual regexp special characters are not special inside a character alternative. A completely different set of characters is special: @samp{]}, @samp{-} and @samp{^}. To include @samp{]} in a character alternative, put it at the beginning. To include @samp{^}, put it anywhere but at the beginning. To include @samp{-}, put it at the end. Thus, @samp{[]^-]} matches all three of these special characters. You cannot use @samp{\} to escape these three characters, since @samp{\} is not special here. The following aspects of ranges are specific to Emacs, in that POSIX allows but does not require this behavior and programs other than Emacs may behave differently: @enumerate @item If @code{case-fold-search} is non-@code{nil}, @samp{[a-z]} also matches upper-case letters. @item A range is not affected by the locale's collation sequence: it always represents the set of characters with codepoints ranging between those of its bounds, so that @samp{[a-z]} matches only ASCII letters, even outside the C or POSIX locale. @item If the lower bound of a range is greater than its upper bound, the range is empty and represents no characters. Thus, @samp{[z-a]} always fails to match, and @samp{[^z-a]} matches any character, including newline. However, a reversed range should always be from the letter @samp{z} to the letter @samp{a} to make it clear that it is not a typo; for example, @samp{[+-*/]} should be avoided, because it matches only @samp{/} rather than the likely-intended four characters. @end enumerate Some kinds of character alternatives are not the best style even though they have a well-defined meaning in Emacs. They include: @enumerate @item Although a range's bound can be almost any character, it is better style to stay within natural sequences of ASCII letters and digits because most people have not memorized character code tables. For example, @samp{[.-9]} is less clear than @samp{[./0-9]}, and @samp{[`-~]} is less clear than @samp{[`a-z@{|@}~]}. Unicode character escapes can help here; for example, for most programmers @samp{[ก-ฺ฿-๛]} is less clear than @samp{[\u0E01-\u0E3A\u0E3F-\u0E5B]}. @item Although a character alternative can include duplicates, it is better style to avoid them. For example, @samp{[XYa-yYb-zX]} is less clear than @samp{[XYa-z]}. @item Although a range can denote just one, two, or three characters, it is simpler to list the characters. For example, @samp{[a-a0]} is less clear than @samp{[a0]}, @samp{[i-j]} is less clear than @samp{[ij]}, and @samp{[i-k]} is less clear than @samp{[ijk]}. @item Although a @samp{-} can appear at the beginning of a character alternative or as the upper bound of a range, it is better style to put @samp{-} by itself at the end of a character alternative. For example, although @samp{[-a-z]} is valid, @samp{[a-z-]} is better style; and although @samp{[*--]} is valid, @samp{[*+,-]} is clearer. @end enumerate @item @samp{[^ @dots{} ]} @cindex @samp{^} in regexp @samp{[^} begins a @dfn{complemented character alternative}. This matches any character except the ones specified. Thus, @samp{[^a-z0-9A-Z]} matches all characters @emph{except} ASCII letters and digits. @samp{^} is not special in a character alternative unless it is the first character. The character following the @samp{^} is treated as if it were first (in other words, @samp{-} and @samp{]} are not special there). A complemented character alternative can match a newline, unless newline is mentioned as one of the characters not to match. This is in contrast to the handling of regexps in programs such as @code{grep}. You can specify named character classes, just like in character alternatives. For instance, @samp{[^[:ascii:]]} matches any non-@acronym{ASCII} character. @xref{Char Classes}. @item @samp{^} @cindex beginning of line in regexp When matching a buffer, @samp{^} matches the empty string, but only at the beginning of a line in the text being matched (or the beginning of the accessible portion of the buffer). Otherwise it fails to match anything. Thus, @samp{^foo} matches a @samp{foo} that occurs at the beginning of a line. When matching a string instead of a buffer, @samp{^} matches at the beginning of the string or after a newline character. For historical compatibility reasons, @samp{^} can be used only at the beginning of the regular expression, or after @samp{\(}, @samp{\(?:} or @samp{\|}. @item @samp{$} @cindex @samp{$} in regexp @cindex end of line in regexp is similar to @samp{^} but matches only at the end of a line (or the end of the accessible portion of the buffer). Thus, @samp{x+$} matches a string of one @samp{x} or more at the end of a line. When matching a string instead of a buffer, @samp{$} matches at the end of the string or before a newline character. For historical compatibility reasons, @samp{$} can be used only at the end of the regular expression, or before @samp{\)} or @samp{\|}. @item @samp{\} @cindex @samp{\} in regexp has two functions: it quotes the special characters (including @samp{\}), and it introduces additional special constructs. Because @samp{\} quotes special characters, @samp{\$} is a regular expression that matches only @samp{$}, and @samp{\[} is a regular expression that matches only @samp{[}, and so on. Note that @samp{\} also has special meaning in the read syntax of Lisp strings (@pxref{String Type}), and must be quoted with @samp{\}. For example, the regular expression that matches the @samp{\} character is @samp{\\}. To write a Lisp string that contains the characters @samp{\\}, Lisp syntax requires you to quote each @samp{\} with another @samp{\}. Therefore, the read syntax for a regular expression matching @samp{\} is @code{"\\\\"}. @end table @strong{Please note:} For historical compatibility, special characters are treated as ordinary ones if they are in contexts where their special meanings make no sense. For example, @samp{*foo} treats @samp{*} as ordinary since there is no preceding expression on which the @samp{*} can act. It is poor practice to depend on this behavior; quote the special character anyway, regardless of where it appears. As a @samp{\} is not special inside a character alternative, it can never remove the special meaning of @samp{-} or @samp{]}. So you should not quote these characters when they have no special meaning either. This would not clarify anything, since backslashes can legitimately precede these characters where they @emph{have} special meaning, as in @samp{[^\]} (@code{"[^\\]"} for Lisp string syntax), which matches any single character except a backslash. In practice, most @samp{]} that occur in regular expressions close a character alternative and hence are special. However, occasionally a regular expression may try to match a complex pattern of literal @samp{[} and @samp{]}. In such situations, it sometimes may be necessary to carefully parse the regexp from the start to determine which square brackets enclose a character alternative. For example, @samp{[^][]]} consists of the complemented character alternative @samp{[^][]} (which matches any single character that is not a square bracket), followed by a literal @samp{]}. The exact rules are that at the beginning of a regexp, @samp{[} is special and @samp{]} not. This lasts until the first unquoted @samp{[}, after which we are in a character alternative; @samp{[} is no longer special (except when it starts a character class) but @samp{]} is special, unless it immediately follows the special @samp{[} or that @samp{[} followed by a @samp{^}. This lasts until the next special @samp{]} that does not end a character class. This ends the character alternative and restores the ordinary syntax of regular expressions; an unquoted @samp{[} is special again and a @samp{]} not. @node Char Classes @subsubsection Character Classes @cindex character classes in regexp @cindex ascii character class, regexp @cindex alnum character class, regexp @cindex alpha character class, regexp @cindex xdigit character class, regexp Here is a table of the classes you can use in a character alternative, and what they mean: @table @samp @item [:ascii:] This matches any @acronym{ASCII} character (codes 0--127). @item [:alnum:] This matches any letter or digit. For multibyte characters, it matches characters whose Unicode @samp{general-category} property (@pxref{Character Properties}) indicates they are alphabetic or decimal number characters. @item [:alpha:] This matches any letter. For multibyte characters, it matches characters whose Unicode @samp{general-category} property (@pxref{Character Properties}) indicates they are alphabetic characters. @item [:blank:] This matches horizontal whitespace, as defined by Annex C of the Unicode Technical Standard #18. In particular, it matches spaces, tabs, and other characters whose Unicode @samp{general-category} property (@pxref{Character Properties}) indicates they are spacing separators. @item [:cntrl:] This matches any character whose code is in the range 0--31. @item [:digit:] This matches @samp{0} through @samp{9}. Thus, @samp{[-+[:digit:]]} matches any digit, as well as @samp{+} and @samp{-}. @item [:graph:] This matches graphic characters---everything except whitespace, @acronym{ASCII} and non-@acronym{ASCII} control characters, surrogates, and codepoints unassigned by Unicode, as indicated by the Unicode @samp{general-category} property (@pxref{Character Properties}). @item [:lower:] This matches any lower-case letter, as determined by the current case table (@pxref{Case Tables}). If @code{case-fold-search} is non-@code{nil}, this also matches any upper-case letter. @item [:multibyte:] This matches any multibyte character (@pxref{Text Representations}). @item [:nonascii:] This matches any non-@acronym{ASCII} character. @item [:print:] This matches any printing character---either whitespace, or a graphic character matched by @samp{[:graph:]}. @item [:punct:] This matches any punctuation character. (At present, for multibyte characters, it matches anything that has non-word syntax.) @item [:space:] This matches any character that has whitespace syntax (@pxref{Syntax Class Table}). @item [:unibyte:] This matches any unibyte character (@pxref{Text Representations}). @item [:upper:] This matches any upper-case letter, as determined by the current case table (@pxref{Case Tables}). If @code{case-fold-search} is non-@code{nil}, this also matches any lower-case letter. @item [:word:] This matches any character that has word syntax (@pxref{Syntax Class Table}). @item [:xdigit:] This matches the hexadecimal digits: @samp{0} through @samp{9}, @samp{a} through @samp{f} and @samp{A} through @samp{F}. @end table @node Regexp Backslash @subsubsection Backslash Constructs in Regular Expressions @cindex backslash in regular expressions For the most part, @samp{\} followed by any character matches only that character. However, there are several exceptions: certain sequences starting with @samp{\} that have special meanings. Here is a table of the special @samp{\} constructs. @table @samp @item \| @cindex @samp{|} in regexp @cindex regexp alternative specifies an alternative. Two regular expressions @var{a} and @var{b} with @samp{\|} in between form an expression that matches anything that either @var{a} or @var{b} matches. Thus, @samp{foo\|bar} matches either @samp{foo} or @samp{bar} but no other string. @samp{\|} applies to the largest possible surrounding expressions. Only a surrounding @samp{\( @dots{} \)} grouping can limit the grouping power of @samp{\|}. If you need full backtracking capability to handle multiple uses of @samp{\|}, use the POSIX regular expression functions (@pxref{POSIX Regexps}). @item \@{@var{m}\@} is a postfix operator that repeats the previous pattern exactly @var{m} times. Thus, @samp{x\@{5\@}} matches the string @samp{xxxxx} and nothing else. @samp{c[ad]\@{3\@}r} matches string such as @samp{caaar}, @samp{cdddr}, @samp{cadar}, and so on. @item \@{@var{m},@var{n}\@} is a more general postfix operator that specifies repetition with a minimum of @var{m} repeats and a maximum of @var{n} repeats. If @var{m} is omitted, the minimum is 0; if @var{n} is omitted, there is no maximum. For both forms, @var{m} and @var{n}, if specified, may be no larger than @ifnottex 2**16 @minus{} 1 @end ifnottex @tex @math{2^{16}-1} @end tex . For example, @samp{c[ad]\@{1,2\@}r} matches the strings @samp{car}, @samp{cdr}, @samp{caar}, @samp{cadr}, @samp{cdar}, and @samp{cddr}, and nothing else.@* @samp{\@{0,1\@}} or @samp{\@{,1\@}} is equivalent to @samp{?}.@* @samp{\@{0,\@}} or @samp{\@{,\@}} is equivalent to @samp{*}.@* @samp{\@{1,\@}} is equivalent to @samp{+}. @item \( @dots{} \) @cindex @samp{(} in regexp @cindex @samp{)} in regexp @cindex regexp grouping is a grouping construct that serves three purposes: @enumerate @item To enclose a set of @samp{\|} alternatives for other operations. Thus, the regular expression @samp{\(foo\|bar\)x} matches either @samp{foox} or @samp{barx}. @item To enclose a complicated expression for the postfix operators @samp{*}, @samp{+} and @samp{?} to operate on. Thus, @samp{ba\(na\)*} matches @samp{ba}, @samp{bana}, @samp{banana}, @samp{bananana}, etc., with any number (zero or more) of @samp{na} strings. @item To record a matched substring for future reference with @samp{\@var{digit}} (see below). @end enumerate This last application is not a consequence of the idea of a parenthetical grouping; it is a separate feature that was assigned as a second meaning to the same @samp{\( @dots{} \)} construct because, in practice, there was usually no conflict between the two meanings. But occasionally there is a conflict, and that led to the introduction of shy groups. @item \(?: @dots{} \) @cindex shy groups @cindex non-capturing group @cindex unnumbered group @cindex @samp{(?:} in regexp is the @dfn{shy group} construct. A shy group serves the first two purposes of an ordinary group (controlling the nesting of other operators), but it does not get a number, so you cannot refer back to its value with @samp{\@var{digit}}. Shy groups are particularly useful for mechanically-constructed regular expressions, because they can be added automatically without altering the numbering of ordinary, non-shy groups. Shy groups are also called @dfn{non-capturing} or @dfn{unnumbered groups}. @item \(?@var{num}: @dots{} \) is the @dfn{explicitly numbered group} construct. Normal groups get their number implicitly, based on their position, which can be inconvenient. This construct allows you to force a particular group number. There is no particular restriction on the numbering, e.g., you can have several groups with the same number in which case the last one to match (i.e., the rightmost match) will win. Implicitly numbered groups always get the smallest integer larger than the one of any previous group. @item \@var{digit} matches the same text that matched the @var{digit}th occurrence of a grouping (@samp{\( @dots{} \)}) construct. In other words, after the end of a group, the matcher remembers the beginning and end of the text matched by that group. Later on in the regular expression you can use @samp{\} followed by @var{digit} to match that same text, whatever it may have been. The strings matching the first nine grouping constructs appearing in the entire regular expression passed to a search or matching function are assigned numbers 1 through 9 in the order that the open parentheses appear in the regular expression. So you can use @samp{\1} through @samp{\9} to refer to the text matched by the corresponding grouping constructs. For example, @samp{\(.*\)\1} matches any newline-free string that is composed of two identical halves. The @samp{\(.*\)} matches the first half, which may be anything, but the @samp{\1} that follows must match the same exact text. If a @samp{\( @dots{} \)} construct matches more than once (which can happen, for instance, if it is followed by @samp{*}), only the last match is recorded. If a particular grouping construct in the regular expression was never matched---for instance, if it appears inside of an alternative that wasn't used, or inside of a repetition that repeated zero times---then the corresponding @samp{\@var{digit}} construct never matches anything. To use an artificial example, @samp{\(foo\(b*\)\|lose\)\2} cannot match @samp{lose}: the second alternative inside the larger group matches it, but then @samp{\2} is undefined and can't match anything. But it can match @samp{foobb}, because the first alternative matches @samp{foob} and @samp{\2} matches @samp{b}. @item \w @cindex @samp{\w} in regexp matches any word-constituent character. The editor syntax table determines which characters these are. @xref{Syntax Tables}. @item \W @cindex @samp{\W} in regexp matches any character that is not a word constituent. @item \s@var{code} @cindex @samp{\s} in regexp matches any character whose syntax is @var{code}. Here @var{code} is a character that represents a syntax code: thus, @samp{w} for word constituent, @samp{-} for whitespace, @samp{(} for open parenthesis, etc. To represent whitespace syntax, use either @samp{-} or a space character. @xref{Syntax Class Table}, for a list of syntax codes and the characters that stand for them. @item \S@var{code} @cindex @samp{\S} in regexp matches any character whose syntax is not @var{code}. @cindex category, regexp search for @item \c@var{c} matches any character whose category is @var{c}. Here @var{c} is a character that represents a category: thus, @samp{c} for Chinese characters or @samp{g} for Greek characters in the standard category table. You can see the list of all the currently defined categories with @kbd{M-x describe-categories @key{RET}}. You can also define your own categories in addition to the standard ones using the @code{define-category} function (@pxref{Categories}). @item \C@var{c} matches any character whose category is not @var{c}. @end table The following regular expression constructs match the empty string---that is, they don't use up any characters---but whether they match depends on the context. For all, the beginning and end of the accessible portion of the buffer are treated as if they were the actual beginning and end of the buffer. @table @samp @item \` @cindex @samp{\`} in regexp matches the empty string, but only at the beginning of the buffer or string being matched against. @item \' @cindex @samp{\'} in regexp matches the empty string, but only at the end of the buffer or string being matched against. @item \= @cindex @samp{\=} in regexp matches the empty string, but only at point. (This construct is not defined when matching against a string.) @item \b @cindex @samp{\b} in regexp matches the empty string, but only at the beginning or end of a word. Thus, @samp{\bfoo\b} matches any occurrence of @samp{foo} as a separate word. @samp{\bballs?\b} matches @samp{ball} or @samp{balls} as a separate word. @samp{\b} matches at the beginning or end of the buffer (or string) regardless of what text appears next to it. @item \B @cindex @samp{\B} in regexp matches the empty string, but @emph{not} at the beginning or end of a word, nor at the beginning or end of the buffer (or string). @item \< @cindex @samp{\<} in regexp matches the empty string, but only at the beginning of a word. @samp{\<} matches at the beginning of the buffer (or string) only if a word-constituent character follows. @item \> @cindex @samp{\>} in regexp matches the empty string, but only at the end of a word. @samp{\>} matches at the end of the buffer (or string) only if the contents end with a word-constituent character. @item \_< @cindex @samp{\_<} in regexp matches the empty string, but only at the beginning of a symbol. A symbol is a sequence of one or more word or symbol constituent characters. @samp{\_<} matches at the beginning of the buffer (or string) only if a symbol-constituent character follows. @item \_> @cindex @samp{\_>} in regexp matches the empty string, but only at the end of a symbol. @samp{\_>} matches at the end of the buffer (or string) only if the contents end with a symbol-constituent character. @end table @kindex invalid-regexp Not every string is a valid regular expression. For example, a string that ends inside a character alternative without a terminating @samp{]} is invalid, and so is a string that ends with a single @samp{\}. If an invalid regular expression is passed to any of the search functions, an @code{invalid-regexp} error is signaled. @node Regexp Example @subsection Complex Regexp Example Here is a complicated regexp which was formerly used by Emacs to recognize the end of a sentence together with any whitespace that follows. (Nowadays Emacs uses a similar but more complex default regexp constructed by the function @code{sentence-end}. @xref{Standard Regexps}.) Below, we show first the regexp as a string in Lisp syntax (to distinguish spaces from tab characters), and then the result of evaluating it. The string constant begins and ends with a double-quote. @samp{\"} stands for a double-quote as part of the string, @samp{\\} for a backslash as part of the string, @samp{\t} for a tab and @samp{\n} for a newline. @example @group "[.?!][]\"')@}]*\\($\\| $\\|\t\\|@ @ \\)[ \t\n]*" @result{} "[.?!][]\"')@}]*\\($\\| $\\| \\|@ @ \\)[ ]*" @end group @end example @noindent In the output, tab and newline appear as themselves. This regular expression contains four parts in succession and can be deciphered as follows: @table @code @item [.?!] The first part of the pattern is a character alternative that matches any one of three characters: period, question mark, and exclamation mark. The match must begin with one of these three characters. (This is one point where the new default regexp used by Emacs differs from the old. The new value also allows some non-@acronym{ASCII} characters that end a sentence without any following whitespace.) @item []\"')@}]* The second part of the pattern matches any closing braces and quotation marks, zero or more of them, that may follow the period, question mark or exclamation mark. The @code{\"} is Lisp syntax for a double-quote in a string. The @samp{*} at the end indicates that the immediately preceding regular expression (a character alternative, in this case) may be repeated zero or more times. @item \\($\\|@ $\\|\t\\|@ @ \\) The third part of the pattern matches the whitespace that follows the end of a sentence: the end of a line (optionally with a space), or a tab, or two spaces. The double backslashes mark the parentheses and vertical bars as regular expression syntax; the parentheses delimit a group and the vertical bars separate alternatives. The dollar sign is used to match the end of a line. @item [ \t\n]* Finally, the last part of the pattern matches any additional whitespace beyond the minimum needed to end a sentence. @end table @ifnottex In the @code{rx} notation (@pxref{Rx Notation}), the regexp could be written @example @group (rx (any ".?!") ; Punctuation ending sentence. (zero-or-more (any "\"')]@}")) ; Closing quotes or brackets. (or line-end (seq " " line-end) "\t" " ") ; Two spaces. (zero-or-more (any "\t\n "))) ; Optional extra whitespace. @end group @end example Since @code{rx} regexps are just S-expressions, they can be formatted and commented as such. @end ifnottex @ifnottex @node Rx Notation @subsection The @code{rx} Structured Regexp Notation @cindex rx @cindex regexp syntax As an alternative to the string-based syntax, Emacs provides the structured @code{rx} notation based on Lisp S-expressions. This notation is usually easier to read, write and maintain than regexp strings, and can be indented and commented freely. It requires a conversion into string form since that is what regexp functions expect, but that conversion typically takes place during byte-compilation rather than when the Lisp code using the regexp is run. Here is an @code{rx} regexp@footnote{It could be written much simpler with non-greedy operators (how?), but that would make the example less interesting.} that matches a block comment in the C programming language: @example @group (rx "/*" ; Initial /* (zero-or-more (or (not (any "*")) ; Either non-*, (seq "*" ; or * followed by (not (any "/"))))) ; non-/ (one-or-more "*") ; At least one star, "/") ; and the final / @end group @end example @noindent or, using shorter synonyms and written more compactly, @example @group (rx "/*" (* (| (not (any "*")) (: "*" (not (any "/"))))) (+ "*") "/") @end group @end example @noindent In conventional string syntax, it would be written @example "/\\*\\(?:[^*]\\|\\*[^/]\\)*\\*+/" @end example The @code{rx} notation is mainly useful in Lisp code; it cannot be used in most interactive situations where a regexp is requested, such as when running @code{query-replace-regexp} or in variable customisation. @menu * Rx Constructs:: Constructs valid in rx forms. * Rx Functions:: Functions and macros that use rx forms. @end menu @node Rx Constructs @subsubsection Constructs in @code{rx} regexps The various forms in @code{rx} regexps are described below. The shorthand @var{rx} represents any @code{rx} form, and @var{rx}@dots{} means one or more @code{rx} forms. Where the corresponding string regexp syntax is given, @var{A}, @var{B}, @dots{} are string regexp subexpressions. @c With the new implementation of rx, this can be changed from @c 'one or more' to 'zero or more'. @subsubheading Literals @table @asis @item @code{"some-string"} Match the string @samp{some-string} literally. There are no characters with special meaning, unlike in string regexps. @item @code{?C} Match the character @samp{C} literally. @end table @subsubheading Sequence and alternative @table @asis @item @code{(seq @var{rx}@dots{})} @cindex @code{seq} in rx @itemx @code{(sequence @var{rx}@dots{})} @cindex @code{sequence} in rx @itemx @code{(: @var{rx}@dots{})} @cindex @code{:} in rx @itemx @code{(and @var{rx}@dots{})} @cindex @code{and} in rx Match the @var{rx}s in sequence. Without arguments, the expression matches the empty string.@* Corresponding string regexp: @samp{@var{A}@var{B}@dots{}} (subexpressions in sequence). @item @code{(or @var{rx}@dots{})} @cindex @code{or} in rx @itemx @code{(| @var{rx}@dots{})} @cindex @code{|} in rx Match exactly one of the @var{rx}s, trying from left to right. Without arguments, the expression will not match anything at all.@* Corresponding string regexp: @samp{@var{A}\|@var{B}\|@dots{}}. @end table @subsubheading Repetition Normally, repetition forms are greedy, in that they attempt to match as many times as possible. Some forms are non-greedy; they try to match as few times as possible (@pxref{Non-greedy repetition}). @table @code @item (zero-or-more @var{rx}@dots{}) @cindex @code{zero-or-more} in rx @itemx (0+ @var{rx}@dots{}) @cindex @code{0+} in rx Match the @var{rx}s zero or more times. Greedy by default.@* Corresponding string regexp: @samp{@var{A}*} (greedy), @samp{@var{A}*?} (non-greedy) @item (one-or-more @var{rx}@dots{}) @cindex @code{one-or-more} in rx @itemx (1+ @var{rx}@dots{}) @cindex @code{1+} in rx Match the @var{rx}s one or more times. Greedy by default.@* Corresponding string regexp: @samp{@var{A}+} (greedy), @samp{@var{A}+?} (non-greedy) @item (zero-or-one @var{rx}@dots{}) @cindex @code{zero-or-one} in rx @itemx (optional @var{rx}@dots{}) @cindex @code{optional} in rx @itemx (opt @var{rx}@dots{}) @cindex @code{opt} in rx Match the @var{rx}s once or an empty string. Greedy by default.@* Corresponding string regexp: @samp{@var{A}?} (greedy), @samp{@var{A}??} (non-greedy). @item (* @var{rx}@dots{}) @cindex @code{*} in rx Match the @var{rx}s zero or more times. Greedy.@* Corresponding string regexp: @samp{@var{A}*} @item (+ @var{rx}@dots{}) @cindex @code{+} in rx Match the @var{rx}s one or more times. Greedy.@* Corresponding string regexp: @samp{@var{A}+} @item (? @var{rx}@dots{}) @cindex @code{?} in rx Match the @var{rx}s once or an empty string. Greedy.@* Corresponding string regexp: @samp{@var{A}?} @item (*? @var{rx}@dots{}) @cindex @code{*?} in rx Match the @var{rx}s zero or more times. Non-greedy.@* Corresponding string regexp: @samp{@var{A}*?} @item (+? @var{rx}@dots{}) @cindex @code{+?} in rx Match the @var{rx}s one or more times. Non-greedy.@* Corresponding string regexp: @samp{@var{A}+?} @item (?? @var{rx}@dots{}) @cindex @code{??} in rx Match the @var{rx}s or an empty string. Non-greedy.@* Corresponding string regexp: @samp{@var{A}??} @item (= @var{n} @var{rx}@dots{}) @cindex @code{=} in rx @itemx (repeat @var{n} @var{rx}) Match the @var{rx}s exactly @var{n} times.@* Corresponding string regexp: @samp{@var{A}\@{@var{n}\@}} @item (>= @var{n} @var{rx}@dots{}) @cindex @code{>=} in rx Match the @var{rx}s @var{n} or more times. Greedy.@* Corresponding string regexp: @samp{@var{A}\@{@var{n},\@}} @item (** @var{n} @var{m} @var{rx}@dots{}) @cindex @code{**} in rx @itemx (repeat @var{n} @var{m} @var{rx}@dots{}) @cindex @code{repeat} in rx Match the @var{rx}s at least @var{n} but no more than @var{m} times. Greedy.@* Corresponding string regexp: @samp{@var{A}\@{@var{n},@var{m}\@}} @end table The greediness of some repetition forms can be controlled using the following constructs. However, it is usually better to use the explicit non-greedy forms above when such matching is required. @table @code @item (minimal-match @var{rx}) @cindex @code{minimal-match} in rx Match @var{rx}, with @code{zero-or-more}, @code{0+}, @code{one-or-more}, @code{1+}, @code{zero-or-one}, @code{opt} and @code{optional} using non-greedy matching. @item (maximal-match @var{rx}) @cindex @code{maximal-match} in rx Match @var{rx}, with @code{zero-or-more}, @code{0+}, @code{one-or-more}, @code{1+}, @code{zero-or-one}, @code{opt} and @code{optional} using greedy matching. This is the default. @end table @subsubheading Matching single characters @table @asis @item @code{(any @var{set}@dots{})} @cindex @code{any} in rx @itemx @code{(char @var{set}@dots{})} @cindex @code{char} in rx @itemx @code{(in @var{set}@dots{})} @cindex @code{in} in rx @cindex character class in rx Match a single character from one of the @var{set}s. Each @var{set} is a character, a string representing the set of its characters, a range or a character class (see below). A range is either a hyphen-separated string like @code{"A-Z"}, or a cons of characters like @code{(?A . ?Z)}. Note that hyphen (@code{-}) is special in strings in this construct, since it acts as a range separator. To include a hyphen, add it as a separate character or single-character string.@* Corresponding string regexp: @samp{[@dots{}]} @item @code{(not @var{charspec})} @cindex @code{not} in rx Match a character not included in @var{charspec}. @var{charspec} can be an @code{any}, @code{syntax} or @code{category} form, or a character class.@* Corresponding string regexp: @samp{[^@dots{}]}, @samp{\S@var{code}}, @samp{\C@var{code}} @item @code{not-newline}, @code{nonl} @cindex @code{not-newline} in rx @cindex @code{nonl} in rx Match any character except a newline.@* Corresponding string regexp: @samp{.} (dot) @item @code{anything} @cindex @code{anything} in rx Match any character.@* Corresponding string regexp: @samp{.\|\n} (for example) @item character class @cindex character class in rx Match a character from a named character class: @table @asis @item @code{alpha}, @code{alphabetic}, @code{letter} Match alphabetic characters. More precisely, match characters whose Unicode @samp{general-category} property indicates that they are alphabetic. @item @code{alnum}, @code{alphanumeric} Match alphabetic characters and digits. More precisely, match characters whose Unicode @samp{general-category} property indicates that they are alphabetic or decimal digits. @item @code{digit}, @code{numeric}, @code{num} Match the digits @samp{0}--@samp{9}. @item @code{xdigit}, @code{hex-digit}, @code{hex} Match the hexadecimal digits @samp{0}--@samp{9}, @samp{A}--@samp{F} and @samp{a}--@samp{f}. @item @code{cntrl}, @code{control} Match any character whose code is in the range 0--31. @item @code{blank} Match horizontal whitespace. More precisely, match characters whose Unicode @samp{general-category} property indicates that they are spacing separators. @item @code{space}, @code{whitespace}, @code{white} Match any character that has whitespace syntax (@pxref{Syntax Class Table}). @item @code{lower}, @code{lower-case} Match anything lower-case, as determined by the current case table. If @code{case-fold-search} is non-nil, this also matches any upper-case letter. @item @code{upper}, @code{upper-case} Match anything upper-case, as determined by the current case table. If @code{case-fold-search} is non-nil, this also matches any lower-case letter. @item @code{graph}, @code{graphic} Match any character except whitespace, @acronym{ASCII} and non-@acronym{ASCII} control characters, surrogates, and codepoints unassigned by Unicode, as indicated by the Unicode @samp{general-category} property. @item @code{print}, @code{printing} Match whitespace or a character matched by @code{graph}. @item @code{punct}, @code{punctuation} Match any punctuation character. (At present, for multibyte characters, anything that has non-word syntax.) @item @code{word}, @code{wordchar} Match any character that has word syntax (@pxref{Syntax Class Table}). @item @code{ascii} Match any @acronym{ASCII} character (codes 0--127). @item @code{nonascii} Match any non-@acronym{ASCII} character (but not raw bytes). @end table Corresponding string regexp: @samp{[[:@var{class}:]]} @item @code{(syntax @var{syntax})} @cindex @code{syntax} in rx Match a character with syntax @var{syntax}, being one of the following names: @multitable {@code{close-parenthesis}} {Syntax character} @headitem Syntax name @tab Syntax character @item @code{whitespace} @tab @code{-} @item @code{punctuation} @tab @code{.} @item @code{word} @tab @code{w} @item @code{symbol} @tab @code{_} @item @code{open-parenthesis} @tab @code{(} @item @code{close-parenthesis} @tab @code{)} @item @code{expression-prefix} @tab @code{'} @item @code{string-quote} @tab @code{"} @item @code{paired-delimiter} @tab @code{$} @item @code{escape} @tab @code{\} @item @code{character-quote} @tab @code{/} @item @code{comment-start} @tab @code{<} @item @code{comment-end} @tab @code{>} @item @code{string-delimiter} @tab @code{|} @item @code{comment-delimiter} @tab @code{!} @end multitable For details, @pxref{Syntax Class Table}. Please note that @code{(syntax punctuation)} is @emph{not} equivalent to the character class @code{punctuation}.@* Corresponding string regexp: @samp{\s@var{code}} @item @code{(category @var{category})} @cindex @code{category} in rx Match a character in category @var{category}, which is either one of the names below or its category character. @multitable {@code{vowel-modifying-diacritical-mark}} {Category character} @headitem Category name @tab Category character @item @code{space-for-indent} @tab space @item @code{base} @tab @code{.} @item @code{consonant} @tab @code{0} @item @code{base-vowel} @tab @code{1} @item @code{upper-diacritical-mark} @tab @code{2} @item @code{lower-diacritical-mark} @tab @code{3} @item @code{tone-mark} @tab @code{4} @item @code{symbol} @tab @code{5} @item @code{digit} @tab @code{6} @item @code{vowel-modifying-diacritical-mark} @tab @code{7} @item @code{vowel-sign} @tab @code{8} @item @code{semivowel-lower} @tab @code{9} @item @code{not-at-end-of-line} @tab @code{<} @item @code{not-at-beginning-of-line} @tab @code{>} @item @code{alpha-numeric-two-byte} @tab @code{A} @item @code{chinese-two-byte} @tab @code{C} @item @code{greek-two-byte} @tab @code{G} @item @code{japanese-hiragana-two-byte} @tab @code{H} @item @code{indian-two-byte} @tab @code{I} @item @code{japanese-katakana-two-byte} @tab @code{K} @item @code{strong-left-to-right} @tab @code{L} @item @code{korean-hangul-two-byte} @tab @code{N} @item @code{strong-right-to-left} @tab @code{R} @item @code{cyrillic-two-byte} @tab @code{Y} @item @code{combining-diacritic} @tab @code{^} @item @code{ascii} @tab @code{a} @item @code{arabic} @tab @code{b} @item @code{chinese} @tab @code{c} @item @code{ethiopic} @tab @code{e} @item @code{greek} @tab @code{g} @item @code{korean} @tab @code{h} @item @code{indian} @tab @code{i} @item @code{japanese} @tab @code{j} @item @code{japanese-katakana} @tab @code{k} @item @code{latin} @tab @code{l} @item @code{lao} @tab @code{o} @item @code{tibetan} @tab @code{q} @item @code{japanese-roman} @tab @code{r} @item @code{thai} @tab @code{t} @item @code{vietnamese} @tab @code{v} @item @code{hebrew} @tab @code{w} @item @code{cyrillic} @tab @code{y} @item @code{can-break} @tab @code{|} @end multitable For more information about currently defined categories, run the command @kbd{M-x describe-categories @key{RET}}. For how to define new categories, @pxref{Categories}.@* Corresponding string regexp: @samp{\c@var{code}} @end table @subsubheading Zero-width assertions These all match the empty string, but only in specific places. @table @asis @item @code{line-start}, @code{bol} @cindex @code{line-start} in rx @cindex @code{bol} in rx Match at the beginning of a line.@* Corresponding string regexp: @samp{^} @item @code{line-end}, @code{eol} @cindex @code{line-end} in rx @cindex @code{eol} in rx Match at the end of a line.@* Corresponding string regexp: @samp{$} @item @code{string-start}, @code{bos}, @code{buffer-start}, @code{bot} @cindex @code{string-start} in rx @cindex @code{bos} in rx @cindex @code{buffer-start} in rx @cindex @code{bot} in rx Match at the start of the string or buffer being matched against.@* Corresponding string regexp: @samp{\`} @item @code{string-end}, @code{eos}, @code{buffer-end}, @code{eot} @cindex @code{string-end} in rx @cindex @code{eos} in rx @cindex @code{buffer-end} in rx @cindex @code{eot} in rx Match at the end of the string or buffer being matched against.@* Corresponding string regexp: @samp{\'} @item @code{point} @cindex @code{point} in rx Match at point.@* Corresponding string regexp: @samp{\=} @item @code{word-start} @cindex @code{word-start} in rx Match at the beginning of a word.@* Corresponding string regexp: @samp{\<} @item @code{word-end} @cindex @code{word-end} in rx Match at the end of a word.@* Corresponding string regexp: @samp{\>} @item @code{word-boundary} @cindex @code{word-boundary} in rx Match at the beginning or end of a word.@* Corresponding string regexp: @samp{\b} @item @code{not-word-boundary} @cindex @code{not-word-boundary} in rx Match anywhere but at the beginning or end of a word.@* Corresponding string regexp: @samp{\B} @item @code{symbol-start} @cindex @code{symbol-start} in rx Match at the beginning of a symbol.@* Corresponding string regexp: @samp{\_<} @item @code{symbol-end} @cindex @code{symbol-end} in rx Match at the end of a symbol.@* Corresponding string regexp: @samp{\_>} @end table @subsubheading Capture groups @table @code @item (group @var{rx}@dots{}) @cindex @code{group} in rx @itemx (submatch @var{rx}@dots{}) @cindex @code{submatch} in rx Match the @var{rx}s, making the matched text and position accessible in the match data. The first group in a regexp is numbered 1; subsequent groups will be numbered one higher than the previous group.@* Corresponding string regexp: @samp{\(@dots{}\)} @item (group-n @var{n} @var{rx}@dots{}) @cindex @code{group-n} in rx @itemx (submatch-n @var{n} @var{rx}@dots{}) @cindex @code{submatch-n} in rx Like @code{group}, but explicitly assign the group number @var{n}. @var{n} must be positive.@* Corresponding string regexp: @samp{\(?@var{n}:@dots{}\)} @item (backref @var{n}) @cindex @code{backref} in rx Match the text previously matched by group number @var{n}. @var{n} must be in the range 1--9.@* Corresponding string regexp: @samp{\@var{n}} @end table @subsubheading Dynamic inclusion @table @code @item (literal @var{expr}) @cindex @code{literal} in rx Match the literal string that is the result from evaluating the Lisp expression @var{expr}. The evaluation takes place at call time, in the current lexical environment. @item (regexp @var{expr}) @cindex @code{regexp} in rx @itemx (regex @var{expr}) @cindex @code{regex} in rx Match the string regexp that is the result from evaluating the Lisp expression @var{expr}. The evaluation takes place at call time, in the current lexical environment. @item (eval @var{expr}) @cindex @code{eval} in rx Match the rx form that is the result from evaluating the Lisp expression @var{expr}. The evaluation takes place at macro-expansion time for @code{rx}, at call time for @code{rx-to-string}, in the current global environment. @end table @node Rx Functions @subsubsection Functions and macros using @code{rx} regexps @defmac rx rx-expr@dots{} Translate the @var{rx-expr}s to a string regexp, as if they were the body of a @code{(seq @dots{})} form. The @code{rx} macro expands to a string constant, or, if @code{literal} or @code{regexp} forms are used, a Lisp expression that evaluates to a string. @end defmac @defun rx-to-string rx-expr &optional no-group Translate @var{rx-expr} to a string regexp which is returned. If @var{no-group} is absent or nil, bracket the result in a non-capturing group, @samp{\(?:@dots{}\)}, if necessary to ensure that a postfix operator appended to it will apply to the whole expression. Arguments to @code{literal} and @code{regexp} forms in @var{rx-expr} must be string literals. @end defun The @code{pcase} macro can use @code{rx} expressions as patterns directly; @pxref{rx in pcase}. @end ifnottex @node Regexp Functions @subsection Regular Expression Functions These functions operate on regular expressions. @cindex quote special characters in regexp @defun regexp-quote string This function returns a regular expression whose only exact match is @var{string}. Using this regular expression in @code{looking-at} will succeed only if the next characters in the buffer are @var{string}; using it in a search function will succeed if the text being searched contains @var{string}. @xref{Regexp Search}. This allows you to request an exact string match or search when calling a function that wants a regular expression. @example @group (regexp-quote "^The cat$") @result{} "\\^The cat\\$" @end group @end example One use of @code{regexp-quote} is to combine an exact string match with context described as a regular expression. For example, this searches for the string that is the value of @var{string}, surrounded by whitespace: @example @group (re-search-forward (concat "\\s-" (regexp-quote string) "\\s-")) @end group @end example @end defun @cindex optimize regexp @defun regexp-opt strings &optional paren keep-order This function returns an efficient regular expression that will match any of the strings in the list @var{strings}. This is useful when you need to make matching or searching as fast as possible---for example, for Font Lock mode@footnote{Note that @code{regexp-opt} does not guarantee that its result is absolutely the most efficient form possible. A hand-tuned regular expression can sometimes be slightly more efficient, but is almost never worth the effort.}. @c E.g., see https://debbugs.gnu.org/2816 If @var{strings} is the empty list, the return value is a regexp that never matches anything. The optional argument @var{paren} can be any of the following: @table @asis @item a string The resulting regexp is preceded by @var{paren} and followed by @samp{\)}, e.g. use @samp{"\\(?1:"} to produce an explicitly numbered group. @item @code{words} The resulting regexp is surrounded by @samp{\<\(} and @samp{\)\>}. @item @code{symbols} The resulting regexp is surrounded by @samp{\_<\(} and @samp{\)\_>} (this is often appropriate when matching programming-language keywords and the like). @item non-@code{nil} The resulting regexp is surrounded by @samp{\(} and @samp{\)}. @item @code{nil} The resulting regexp is surrounded by @samp{\(?:} and @samp{\)}, if it is necessary to ensure that a postfix operator appended to it will apply to the whole expression. @end table The optional argument @var{keep-order}, if @code{nil} or omitted, allows the returned regexp to match the strings in any order. If non-@code{nil}, the match is guaranteed to be performed in the order given, as if the strings were made into a regexp by joining them with the @samp{\|} operator. Up to reordering, the resulting regexp of @code{regexp-opt} is equivalent to but usually more efficient than that of a simplified version: @example (defun simplified-regexp-opt (strings &optional paren) (let ((parens (cond ((stringp paren) (cons paren "\\)")) ((eq paren 'words) '("\\<\\(" . "\\)\\>")) ((eq paren 'symbols) '("\\_<\\(" . "\\)\\_>")) ((null paren) '("\\(?:" . "\\)")) (t '("\\(" . "\\)"))))) (concat (car parens) (mapconcat 'regexp-quote strings "\\|") (cdr parens)))) @end example @end defun @defun regexp-opt-depth regexp This function returns the total number of grouping constructs (parenthesized expressions) in @var{regexp}. This does not include shy groups (@pxref{Regexp Backslash}). @end defun @c Supposedly an internal regexp-opt function, but table.el uses it at least. @defun regexp-opt-charset chars This function returns a regular expression matching a character in the list of characters @var{chars}. @example (regexp-opt-charset '(?a ?b ?c ?d ?e)) @result{} "[a-e]" @end example @end defun @c Internal functions: regexp-opt-group @defvar regexp-unmatchable This variable contains a regexp that is guaranteed not to match any string at all. It is particularly useful as default value for variables that may be set to a pattern that actually matches something. @end defvar @node Regexp Search @section Regular Expression Searching @cindex regular expression searching @cindex regexp searching @cindex searching for regexp In GNU Emacs, you can search for the next match for a regular expression (@pxref{Syntax of Regexps}) either incrementally or not. For incremental search commands, see @ref{Regexp Search, , Regular Expression Search, emacs, The GNU Emacs Manual}. Here we describe only the search functions useful in programs. The principal one is @code{re-search-forward}. These search functions convert the regular expression to multibyte if the buffer is multibyte; they convert the regular expression to unibyte if the buffer is unibyte. @xref{Text Representations}. @deffn Command re-search-forward regexp &optional limit noerror count This function searches forward in the current buffer for a string of text that is matched by the regular expression @var{regexp}. The function skips over any amount of text that is not matched by @var{regexp}, and leaves point at the end of the first match found. It returns the new value of point. If @var{limit} is non-@code{nil}, it must be a position in the current buffer. It specifies the upper bound to the search. No match extending after that position is accepted. If @var{limit} is omitted or @code{nil}, it defaults to the end of the accessible portion of the buffer. What @code{re-search-forward} does when the search fails depends on the value of @var{noerror}: @table @asis @item @code{nil} Signal a @code{search-failed} error. @item @code{t} Do nothing and return @code{nil}. @item anything else Move point to @var{limit} (or the end of the accessible portion of the buffer) and return @code{nil}. @end table The argument @var{noerror} only affects valid searches which fail to find a match. Invalid arguments cause errors regardless of @var{noerror}. If @var{count} is a positive number @var{n}, the search is done @var{n} times; each successive search starts at the end of the previous match. If all these successive searches succeed, the function call succeeds, moving point and returning its new value. Otherwise the function call fails, with results depending on the value of @var{noerror}, as described above. If @var{count} is a negative number @minus{}@var{n}, the search is done @var{n} times in the opposite (backward) direction. In the following example, point is initially before the @samp{T}. Evaluating the search call moves point to the end of that line (between the @samp{t} of @samp{hat} and the newline). @example @group ---------- Buffer: foo ---------- I read "@point{}The cat in the hat comes back" twice. ---------- Buffer: foo ---------- @end group @group (re-search-forward "[a-z]+" nil t 5) @result{} 27 ---------- Buffer: foo ---------- I read "The cat in the hat@point{} comes back" twice. ---------- Buffer: foo ---------- @end group @end example @end deffn @c This anchor is referenced by re-search-backward's docstring. @anchor{re-search-backward} @deffn Command re-search-backward regexp &optional limit noerror count This function searches backward in the current buffer for a string of text that is matched by the regular expression @var{regexp}, leaving point at the beginning of the first text found. This function is analogous to @code{re-search-forward}, but they are not simple mirror images. @code{re-search-forward} finds the match whose beginning is as close as possible to the starting point. If @code{re-search-backward} were a perfect mirror image, it would find the match whose end is as close as possible. However, in fact it finds the match whose beginning is as close as possible (and yet ends before the starting point). The reason for this is that matching a regular expression at a given spot always works from beginning to end, and starts at a specified beginning position. A true mirror-image of @code{re-search-forward} would require a special feature for matching regular expressions from end to beginning. It's not worth the trouble of implementing that. @end deffn @defun string-match regexp string &optional start This function returns the index of the start of the first match for the regular expression @var{regexp} in @var{string}, or @code{nil} if there is no match. If @var{start} is non-@code{nil}, the search starts at that index in @var{string}. For example, @example @group (string-match "quick" "The quick brown fox jumped quickly.") @result{} 4 @end group @group (string-match "quick" "The quick brown fox jumped quickly." 8) @result{} 27 @end group @end example @noindent The index of the first character of the string is 0, the index of the second character is 1, and so on. If this function finds a match, the index of the first character beyond the match is available as @code{(match-end 0)}. @xref{Match Data}. @example @group (string-match "quick" "The quick brown fox jumped quickly." 8) @result{} 27 @end group @group (match-end 0) @result{} 32 @end group @end example @end defun @defun string-match-p regexp string &optional start This predicate function does what @code{string-match} does, but it avoids modifying the match data. @end defun @defun looking-at regexp This function determines whether the text in the current buffer directly following point matches the regular expression @var{regexp}. ``Directly following'' means precisely that: the search is ``anchored'' and it can succeed only starting with the first character following point. The result is @code{t} if so, @code{nil} otherwise. This function does not move point, but it does update the match data. @xref{Match Data}. If you need to test for a match without modifying the match data, use @code{looking-at-p}, described below. In this example, point is located directly before the @samp{T}. If it were anywhere else, the result would be @code{nil}. @example @group ---------- Buffer: foo ---------- I read "@point{}The cat in the hat comes back" twice. ---------- Buffer: foo ---------- (looking-at "The cat in the hat$") @result{} t @end group @end example @end defun @defun looking-back regexp limit &optional greedy This function returns @code{t} if @var{regexp} matches the text immediately before point (i.e., ending at point), and @code{nil} otherwise. Because regular expression matching works only going forward, this is implemented by searching backwards from point for a match that ends at point. That can be quite slow if it has to search a long distance. You can bound the time required by specifying a non-@code{nil} value for @var{limit}, which says not to search before @var{limit}. In this case, the match that is found must begin at or after @var{limit}. Here's an example: @example @group ---------- Buffer: foo ---------- I read "@point{}The cat in the hat comes back" twice. ---------- Buffer: foo ---------- (looking-back "read \"" 3) @result{} t (looking-back "read \"" 4) @result{} nil @end group @end example If @var{greedy} is non-@code{nil}, this function extends the match backwards as far as possible, stopping when a single additional previous character cannot be part of a match for @var{regexp}. When the match is extended, its starting position is allowed to occur before @var{limit}. @c https://debbugs.gnu.org/5689 As a general recommendation, try to avoid using @code{looking-back} wherever possible, since it is slow. For this reason, there are no plans to add a @code{looking-back-p} function. @end defun @defun looking-at-p regexp This predicate function works like @code{looking-at}, but without updating the match data. @end defun @defvar search-spaces-regexp If this variable is non-@code{nil}, it should be a regular expression that says how to search for whitespace. In that case, any group of spaces in a regular expression being searched for stands for use of this regular expression. However, spaces inside of constructs such as @samp{[@dots{}]} and @samp{*}, @samp{+}, @samp{?} are not affected by @code{search-spaces-regexp}. Since this variable affects all regular expression search and match constructs, you should bind it temporarily for as small as possible a part of the code. @end defvar @node POSIX Regexps @section POSIX Regular Expression Searching @cindex backtracking and POSIX regular expressions The usual regular expression functions do backtracking when necessary to handle the @samp{\|} and repetition constructs, but they continue this only until they find @emph{some} match. Then they succeed and report the first match found. This section describes alternative search functions which perform the full backtracking specified by the POSIX standard for regular expression matching. They continue backtracking until they have tried all possibilities and found all matches, so they can report the longest match, as required by POSIX@. This is much slower, so use these functions only when you really need the longest match. The POSIX search and match functions do not properly support the non-greedy repetition operators (@pxref{Regexp Special, non-greedy}). This is because POSIX backtracking conflicts with the semantics of non-greedy repetition. @deffn Command posix-search-forward regexp &optional limit noerror count This is like @code{re-search-forward} except that it performs the full backtracking specified by the POSIX standard for regular expression matching. @end deffn @deffn Command posix-search-backward regexp &optional limit noerror count This is like @code{re-search-backward} except that it performs the full backtracking specified by the POSIX standard for regular expression matching. @end deffn @defun posix-looking-at regexp This is like @code{looking-at} except that it performs the full backtracking specified by the POSIX standard for regular expression matching. @end defun @defun posix-string-match regexp string &optional start This is like @code{string-match} except that it performs the full backtracking specified by the POSIX standard for regular expression matching. @end defun @node Match Data @section The Match Data @cindex match data Emacs keeps track of the start and end positions of the segments of text found during a search; this is called the @dfn{match data}. Thanks to the match data, you can search for a complex pattern, such as a date in a mail message, and then extract parts of the match under control of the pattern. Because the match data normally describe the most recent search only, you must be careful not to do another search inadvertently between the search you wish to refer back to and the use of the match data. If you can't avoid another intervening search, you must save and restore the match data around it, to prevent it from being overwritten. Notice that all functions are allowed to overwrite the match data unless they're explicitly documented not to do so. A consequence is that functions that are run implicitly in the background (@pxref{Timers}, and @ref{Idle Timers}) should likely save and restore the match data explicitly. @menu * Replacing Match:: Replacing a substring that was matched. * Simple Match Data:: Accessing single items of match data, such as where a particular subexpression started. * Entire Match Data:: Accessing the entire match data at once, as a list. * Saving Match Data:: Saving and restoring the match data. @end menu @node Replacing Match @subsection Replacing the Text that Matched @cindex replace matched text This function replaces all or part of the text matched by the last search. It works by means of the match data. @cindex case in replacements @defun replace-match replacement &optional fixedcase literal string subexp This function performs a replacement operation on a buffer or string. If you did the last search in a buffer, you should omit the @var{string} argument or specify @code{nil} for it, and make sure that the current buffer is the one in which you performed the last search. Then this function edits the buffer, replacing the matched text with @var{replacement}. It leaves point at the end of the replacement text. If you performed the last search on a string, pass the same string as @var{string}. Then this function returns a new string, in which the matched text is replaced by @var{replacement}. If @var{fixedcase} is non-@code{nil}, then @code{replace-match} uses the replacement text without case conversion; otherwise, it converts the replacement text depending upon the capitalization of the text to be replaced. If the original text is all upper case, this converts the replacement text to upper case. If all words of the original text are capitalized, this capitalizes all the words of the replacement text. If all the words are one-letter and they are all upper case, they are treated as capitalized words rather than all-upper-case words. If @var{literal} is non-@code{nil}, then @var{replacement} is inserted exactly as it is, the only alterations being case changes as needed. If it is @code{nil} (the default), then the character @samp{\} is treated specially. If a @samp{\} appears in @var{replacement}, then it must be part of one of the following sequences: @table @asis @item @samp{\&} @cindex @samp{&} in replacement This stands for the entire text being replaced. @item @samp{\@var{n}}, where @var{n} is a digit @cindex @samp{\@var{n}} in replacement This stands for the text that matched the @var{n}th subexpression in the original regexp. Subexpressions are those expressions grouped inside @samp{\(@dots{}\)}. If the @var{n}th subexpression never matched, an empty string is substituted. @item @samp{\\} @cindex @samp{\} in replacement This stands for a single @samp{\} in the replacement text. @item @samp{\?} This stands for itself (for compatibility with @code{replace-regexp} and related commands; @pxref{Regexp Replace,,, emacs, The GNU Emacs Manual}). @end table @noindent Any other character following @samp{\} signals an error. The substitutions performed by @samp{\&} and @samp{\@var{n}} occur after case conversion, if any. Therefore, the strings they substitute are never case-converted. If @var{subexp} is non-@code{nil}, that says to replace just subexpression number @var{subexp} of the regexp that was matched, not the entire match. For example, after matching @samp{foo \(ba*r\)}, calling @code{replace-match} with 1 as @var{subexp} means to replace just the text that matched @samp{\(ba*r\)}. @end defun @defun match-substitute-replacement replacement &optional fixedcase literal string subexp This function returns the text that would be inserted into the buffer by @code{replace-match}, but without modifying the buffer. It is useful if you want to present the user with actual replacement result, with constructs like @samp{\@var{n}} or @samp{\&} substituted with matched groups. Arguments @var{replacement} and optional @var{fixedcase}, @var{literal}, @var{string} and @var{subexp} have the same meaning as for @code{replace-match}. @end defun @node Simple Match Data @subsection Simple Match Data Access This section explains how to use the match data to find out what was matched by the last search or match operation, if it succeeded. You can ask about the entire matching text, or about a particular parenthetical subexpression of a regular expression. The @var{count} argument in the functions below specifies which. If @var{count} is zero, you are asking about the entire match. If @var{count} is positive, it specifies which subexpression you want. Recall that the subexpressions of a regular expression are those expressions grouped with escaped parentheses, @samp{\(@dots{}\)}. The @var{count}th subexpression is found by counting occurrences of @samp{\(} from the beginning of the whole regular expression. The first subexpression is numbered 1, the second 2, and so on. Only regular expressions can have subexpressions---after a simple string search, the only information available is about the entire match. Every successful search sets the match data. Therefore, you should query the match data immediately after searching, before calling any other function that might perform another search. Alternatively, you may save and restore the match data (@pxref{Saving Match Data}) around the call to functions that could perform another search. Or use the functions that explicitly do not modify the match data; e.g., @code{string-match-p}. @c This is an old comment and presumably there is no prospect of this @c changing now. But still the advice stands. A search which fails may or may not alter the match data. In the current implementation, it does not, but we may change it in the future. Don't try to rely on the value of the match data after a failing search. @defun match-string count &optional in-string This function returns, as a string, the text matched in the last search or match operation. It returns the entire text if @var{count} is zero, or just the portion corresponding to the @var{count}th parenthetical subexpression, if @var{count} is positive. If the last such operation was done against a string with @code{string-match}, then you should pass the same string as the argument @var{in-string}. After a buffer search or match, you should omit @var{in-string} or pass @code{nil} for it; but you should make sure that the current buffer when you call @code{match-string} is the one in which you did the searching or matching. Failure to follow this advice will lead to incorrect results. The value is @code{nil} if @var{count} is out of range, or for a subexpression inside a @samp{\|} alternative that wasn't used or a repetition that repeated zero times. @end defun @defun match-string-no-properties count &optional in-string This function is like @code{match-string} except that the result has no text properties. @end defun @defun match-beginning count If the last regular expression search found a match, this function returns the position of the start of the matching text or of a subexpression of it. If @var{count} is zero, then the value is the position of the start of the entire match. Otherwise, @var{count} specifies a subexpression in the regular expression, and the value of the function is the starting position of the match for that subexpression. The value is @code{nil} for a subexpression inside a @samp{\|} alternative that wasn't used or a repetition that repeated zero times. @end defun @defun match-end count This function is like @code{match-beginning} except that it returns the position of the end of the match, rather than the position of the beginning. @end defun Here is an example of using the match data, with a comment showing the positions within the text: @example @group (string-match "\\(qu\\)\\(ick\\)" "The quick fox jumped quickly.") ;0123456789 @result{} 4 @end group @group (match-string 0 "The quick fox jumped quickly.") @result{} "quick" (match-string 1 "The quick fox jumped quickly.") @result{} "qu" (match-string 2 "The quick fox jumped quickly.") @result{} "ick" @end group @group (match-beginning 1) ; @r{The beginning of the match} @result{} 4 ; @r{with @samp{qu} is at index 4.} @end group @group (match-beginning 2) ; @r{The beginning of the match} @result{} 6 ; @r{with @samp{ick} is at index 6.} @end group @group (match-end 1) ; @r{The end of the match} @result{} 6 ; @r{with @samp{qu} is at index 6.} (match-end 2) ; @r{The end of the match} @result{} 9 ; @r{with @samp{ick} is at index 9.} @end group @end example Here is another example. Point is initially located at the beginning of the line. Searching moves point to between the space and the word @samp{in}. The beginning of the entire match is at the 9th character of the buffer (@samp{T}), and the beginning of the match for the first subexpression is at the 13th character (@samp{c}). @example @group (list (re-search-forward "The \\(cat \\)") (match-beginning 0) (match-beginning 1)) @result{} (17 9 13) @end group @group ---------- Buffer: foo ---------- I read "The cat @point{}in the hat comes back" twice. ^ ^ 9 13 ---------- Buffer: foo ---------- @end group @end example @noindent (In this case, the index returned is a buffer position; the first character of the buffer counts as 1.) @node Entire Match Data @subsection Accessing the Entire Match Data The functions @code{match-data} and @code{set-match-data} read or write the entire match data, all at once. @defun match-data &optional integers reuse reseat This function returns a list of positions (markers or integers) that record all the information on the text that the last search matched. Element zero is the position of the beginning of the match for the whole expression; element one is the position of the end of the match for the expression. The next two elements are the positions of the beginning and end of the match for the first subexpression, and so on. In general, element @ifnottex number 2@var{n} @end ifnottex @tex number {\mathsurround=0pt $2n$} @end tex corresponds to @code{(match-beginning @var{n})}; and element @ifnottex number 2@var{n} + 1 @end ifnottex @tex number {\mathsurround=0pt $2n+1$} @end tex corresponds to @code{(match-end @var{n})}. Normally all the elements are markers or @code{nil}, but if @var{integers} is non-@code{nil}, that means to use integers instead of markers. (In that case, the buffer itself is appended as an additional element at the end of the list, to facilitate complete restoration of the match data.) If the last match was done on a string with @code{string-match}, then integers are always used, since markers can't point into a string. If @var{reuse} is non-@code{nil}, it should be a list. In that case, @code{match-data} stores the match data in @var{reuse}. That is, @var{reuse} is destructively modified. @var{reuse} does not need to have the right length. If it is not long enough to contain the match data, it is extended. If it is too long, the length of @var{reuse} stays the same, but the elements that were not used are set to @code{nil}. The purpose of this feature is to reduce the need for garbage collection. If @var{reseat} is non-@code{nil}, all markers on the @var{reuse} list are reseated to point to nowhere. As always, there must be no possibility of intervening searches between the call to a search function and the call to @code{match-data} that is intended to access the match data for that search. @example @group (match-data) @result{} (# # # #) @end group @end example @end defun @defun set-match-data match-list &optional reseat This function sets the match data from the elements of @var{match-list}, which should be a list that was the value of a previous call to @code{match-data}. (More precisely, anything that has the same format will work.) If @var{match-list} refers to a buffer that doesn't exist, you don't get an error; that sets the match data in a meaningless but harmless way. If @var{reseat} is non-@code{nil}, all markers on the @var{match-list} list are reseated to point to nowhere. @c TODO Make it properly obsolete. @findex store-match-data @code{store-match-data} is a semi-obsolete alias for @code{set-match-data}. @end defun @node Saving Match Data @subsection Saving and Restoring the Match Data When you call a function that may search, you may need to save and restore the match data around that call, if you want to preserve the match data from an earlier search for later use. Here is an example that shows the problem that arises if you fail to save the match data: @example @group (re-search-forward "The \\(cat \\)") @result{} 48 (foo) ; @r{@code{foo} does more searching.} (match-end 0) @result{} 61 ; @r{Unexpected result---not 48!} @end group @end example You can save and restore the match data with @code{save-match-data}: @defmac save-match-data body@dots{} This macro executes @var{body}, saving and restoring the match data around it. The return value is the value of the last form in @var{body}. @end defmac You could use @code{set-match-data} together with @code{match-data} to imitate the effect of the special form @code{save-match-data}. Here is how: @example @group (let ((data (match-data))) (unwind-protect @dots{} ; @r{Ok to change the original match data.} (set-match-data data))) @end group @end example Emacs automatically saves and restores the match data when it runs process filter functions (@pxref{Filter Functions}) and process sentinels (@pxref{Sentinels}). @ignore Here is a function which restores the match data provided the buffer associated with it still exists. @smallexample @group (defun restore-match-data (data) @c It is incorrect to split the first line of a doc string. @c If there's a problem here, it should be solved in some other way. "Restore the match data DATA unless the buffer is missing." (catch 'foo (let ((d data)) @end group (while d (and (car d) (null (marker-buffer (car d))) @group ;; @file{match-data} @r{buffer is deleted.} (throw 'foo nil)) (setq d (cdr d))) (set-match-data data)))) @end group @end smallexample @end ignore @node Search and Replace @section Search and Replace @cindex replacement after search @cindex searching and replacing If you want to find all matches for a regexp in part of the buffer, and replace them, the best way is to write an explicit loop using @code{re-search-forward} and @code{replace-match}, like this: @example (while (re-search-forward "foo[ \t]+bar" nil t) (replace-match "foobar")) @end example @noindent @xref{Replacing Match,, Replacing the Text that Matched}, for a description of @code{replace-match}. However, replacing matches in a string is more complex, especially if you want to do it efficiently. So Emacs provides a function to do this. @defun replace-regexp-in-string regexp rep string &optional fixedcase literal subexp start This function copies @var{string} and searches it for matches for @var{regexp}, and replaces them with @var{rep}. It returns the modified copy. If @var{start} is non-@code{nil}, the search for matches starts at that index in @var{string}, and the returned value does not include the first @var{start} characters of @var{string}. To get the whole transformed string, concatenate the first @var{start} characters of @var{string} with the return value. This function uses @code{replace-match} to do the replacement, and it passes the optional arguments @var{fixedcase}, @var{literal} and @var{subexp} along to @code{replace-match}. Instead of a string, @var{rep} can be a function. In that case, @code{replace-regexp-in-string} calls @var{rep} for each match, passing the text of the match as its sole argument. It collects the value @var{rep} returns and passes that to @code{replace-match} as the replacement string. The match data at this point are the result of matching @var{regexp} against a substring of @var{string}. @end defun If you want to write a command along the lines of @code{query-replace}, you can use @code{perform-replace} to do the work. @defun perform-replace from-string replacements query-flag regexp-flag delimited-flag &optional repeat-count map start end backward region-noncontiguous-p This function is the guts of @code{query-replace} and related commands. It searches for occurrences of @var{from-string} in the text between positions @var{start} and @var{end} and replaces some or all of them. If @var{start} is @code{nil} (or omitted), point is used instead, and the end of the buffer's accessible portion is used for @var{end}. (If the optional argument @var{backward} is non-@code{nil}, the search starts at @var{end} and goes backward.) If @var{query-flag} is @code{nil}, it replaces all occurrences; otherwise, it asks the user what to do about each one. If @var{regexp-flag} is non-@code{nil}, then @var{from-string} is considered a regular expression; otherwise, it must match literally. If @var{delimited-flag} is non-@code{nil}, then only replacements surrounded by word boundaries are considered. The argument @var{replacements} specifies what to replace occurrences with. If it is a string, that string is used. It can also be a list of strings, to be used in cyclic order. If @var{replacements} is a cons cell, @w{@code{(@var{function} . @var{data})}}, this means to call @var{function} after each match to get the replacement text. This function is called with two arguments: @var{data}, and the number of replacements already made. If @var{repeat-count} is non-@code{nil}, it should be an integer. Then it specifies how many times to use each of the strings in the @var{replacements} list before advancing cyclically to the next one. If @var{from-string} contains upper-case letters, then @code{perform-replace} binds @code{case-fold-search} to @code{nil}, and it uses the @var{replacements} without altering their case. Normally, the keymap @code{query-replace-map} defines the possible user responses for queries. The argument @var{map}, if non-@code{nil}, specifies a keymap to use instead of @code{query-replace-map}. Non-@code{nil} @var{region-noncontiguous-p} means that the region between @var{start} and @var{end} is composed of noncontiguous pieces. The most common example of this is a rectangular region, where the pieces are separated by newline characters. This function uses one of two functions to search for the next occurrence of @var{from-string}. These functions are specified by the values of two variables: @code{replace-re-search-function} and @code{replace-search-function}. The former is called when the argument @var{regexp-flag} is non-@code{nil}, the latter when it is @code{nil}. @end defun @defvar query-replace-map This variable holds a special keymap that defines the valid user responses for @code{perform-replace} and the commands that use it, as well as @code{y-or-n-p} and @code{map-y-or-n-p}. This map is unusual in two ways: @itemize @bullet @item The key bindings are not commands, just symbols that are meaningful to the functions that use this map. @item Prefix keys are not supported; each key binding must be for a single-event key sequence. This is because the functions don't use @code{read-key-sequence} to get the input; instead, they read a single event and look it up ``by hand''. @end itemize @end defvar Here are the meaningful bindings for @code{query-replace-map}. Several of them are meaningful only for @code{query-replace} and friends. @table @code @item act Do take the action being considered---in other words, ``yes''. @item skip Do not take action for this question---in other words, ``no''. @item exit Answer this question ``no'', and give up on the entire series of questions, assuming that the answers will be ``no''. @item exit-prefix Like @code{exit}, but add the key that was pressed to @code{unread-command-events} (@pxref{Event Input Misc}). @item act-and-exit Answer this question ``yes'', and give up on the entire series of questions, assuming that subsequent answers will be ``no''. @item act-and-show Answer this question ``yes'', but show the results---don't advance yet to the next question. @item automatic Answer this question and all subsequent questions in the series with ``yes'', without further user interaction. @item backup Move back to the previous place that a question was asked about. @item undo Undo last replacement and move back to the place where that replacement was performed. @item undo-all Undo all replacements and move back to the place where the first replacement was performed. @item edit Enter a recursive edit to deal with this question---instead of any other action that would normally be taken. @item edit-replacement Edit the replacement for this question in the minibuffer. @item delete-and-edit Delete the text being considered, then enter a recursive edit to replace it. @item recenter @itemx scroll-up @itemx scroll-down @itemx scroll-other-window @itemx scroll-other-window-down Perform the specified window scroll operation, then ask the same question again. Only @code{y-or-n-p} and related functions use this answer. @item quit Perform a quit right away. Only @code{y-or-n-p} and related functions use this answer. @item help Display some help, then ask again. @end table @defvar multi-query-replace-map This variable holds a keymap that extends @code{query-replace-map} by providing additional keybindings that are useful in multi-buffer replacements. The additional bindings are: @table @code @item automatic-all Answer this question and all subsequent questions in the series with ``yes'', without further user interaction, for all remaining buffers. @item exit-current Answer this question ``no'', and give up on the entire series of questions for the current buffer. Continue to the next buffer in the sequence. @end table @end defvar @defvar replace-search-function This variable specifies a function that @code{perform-replace} calls to search for the next string to replace. Its default value is @code{search-forward}. Any other value should name a function of 3 arguments: the first 3 arguments of @code{search-forward} (@pxref{String Search}). @end defvar @defvar replace-re-search-function This variable specifies a function that @code{perform-replace} calls to search for the next regexp to replace. Its default value is @code{re-search-forward}. Any other value should name a function of 3 arguments: the first 3 arguments of @code{re-search-forward} (@pxref{Regexp Search}). @end defvar @node Standard Regexps @section Standard Regular Expressions Used in Editing @cindex regexps used standardly in editing @cindex standard regexps used in editing This section describes some variables that hold regular expressions used for certain purposes in editing: @defopt page-delimiter This is the regular expression describing line-beginnings that separate pages. The default value is @code{"^\014"} (i.e., @code{"^^L"} or @code{"^\C-l"}); this matches a line that starts with a formfeed character. @end defopt The following two regular expressions should @emph{not} assume the match always starts at the beginning of a line; they should not use @samp{^} to anchor the match. Most often, the paragraph commands do check for a match only at the beginning of a line, which means that @samp{^} would be superfluous. When there is a nonzero left margin, they accept matches that start after the left margin. In that case, a @samp{^} would be incorrect. However, a @samp{^} is harmless in modes where a left margin is never used. @defopt paragraph-separate This is the regular expression for recognizing the beginning of a line that separates paragraphs. (If you change this, you may have to change @code{paragraph-start} also.) The default value is @w{@code{"[@ \t\f]*$"}}, which matches a line that consists entirely of spaces, tabs, and form feeds (after its left margin). @end defopt @defopt paragraph-start This is the regular expression for recognizing the beginning of a line that starts @emph{or} separates paragraphs. The default value is @w{@code{"\f\\|[ \t]*$"}}, which matches a line containing only whitespace or starting with a form feed (after its left margin). @end defopt @defopt sentence-end If non-@code{nil}, the value should be a regular expression describing the end of a sentence, including the whitespace following the sentence. (All paragraph boundaries also end sentences, regardless.) If the value is @code{nil}, as it is by default, then the function @code{sentence-end} constructs the regexp. That is why you should always call the function @code{sentence-end} to obtain the regexp to be used to recognize the end of a sentence. @end defopt @defun sentence-end This function returns the value of the variable @code{sentence-end}, if non-@code{nil}. Otherwise it returns a default value based on the values of the variables @code{sentence-end-double-space} (@pxref{Definition of sentence-end-double-space}), @code{sentence-end-without-period}, and @code{sentence-end-without-space}. @end defun