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Diffstat (limited to 'pod/perlre.pod')
| -rw-r--r-- | pod/perlre.pod | 77 |
1 files changed, 38 insertions, 39 deletions
diff --git a/pod/perlre.pod b/pod/perlre.pod index c4dbac63c6..ce054ec448 100644 --- a/pod/perlre.pod +++ b/pod/perlre.pod @@ -5,7 +5,7 @@ perlre - Perl regular expressions =head1 DESCRIPTION This page describes the syntax of regular expressions in Perl. For a -description of how to actually I<use> regular expressions in matching +description of how to I<use> regular expressions in matching operations, plus various examples of the same, see C<m//> and C<s///> in L<perlop>. @@ -22,7 +22,7 @@ Do case-insensitive pattern matching. =item m Treat string as multiple lines. That is, change "^" and "$" from matching -only at the very start or end of the string to the start or end of any +at only the very start or end of the string to the start or end of any line anywhere within the string, =item s @@ -45,7 +45,7 @@ The C</x> modifier itself needs a little more explanation. It tells the regular expression parser to ignore whitespace that is neither backslashed nor within a character class. You can use this to break up your regular expression into (slightly) more readable parts. The C<#> -character is also treated as a metacharacter introducing a comment, +character is also treated as a meta-character introducing a comment, just as in ordinary Perl code. This also means that if you want real whitespace or C<#> characters in the pattern that you'll have to either escape them or encode them using octal or hex escapes. Taken together, @@ -63,7 +63,7 @@ See L<Version 8 Regular Expressions> for details. In particular the following metacharacters have their standard I<egrep>-ish meanings: - \ Quote the next metacharacter + \ Quote the next meta-character ^ Match the beginning of the line . Match any character (except newline) $ Match the end of the line (or before newline at the end) @@ -71,8 +71,8 @@ meanings: () Grouping [] Character class -By default, the "^" character is guaranteed to match only at the -beginning of the string, the "$" character only at the end (or before the +By default, the "^" character is guaranteed to match at only the +beginning of the string, the "$" character at only the end (or before the newline at the end) and Perl does certain optimizations with the assumption that the string contains only one line. Embedded newlines will not be matched by "^" or "$". You may, however, wish to treat a @@ -80,7 +80,7 @@ string as a multi-line buffer, such that the "^" will match after any newline within the string, and "$" will match before any newline. At the cost of a little more overhead, you can do this by using the /m modifier on the pattern match operator. (Older programs did this by setting C<$*>, -but this practice is deprecated in Perl 5.) +but this practice is now deprecated.) To facilitate multi-line substitutions, the "." character never matches a newline unless you use the C</s> modifier, which in effect tells Perl to pretend @@ -102,7 +102,7 @@ as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+" modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited to integral values less than 65536. -By default, a quantified subpattern is "greedy", that is, it will match as +By default, a quantified sub-pattern is "greedy", that is, it will match as many times as possible without causing the rest of the pattern not to match. The standard quantifiers are all "greedy", in that they match as many occurrences as possible (given a particular starting location) without @@ -117,7 +117,7 @@ Note that the meanings don't change, just the "gravity": {n,}? Match at least n times {n,m}? Match at least n but not more than m times -Since patterns are processed as double quoted strings, the following +Because patterns are processed as double quoted strings, the following also work: \t tab (HT, TAB) @@ -147,15 +147,15 @@ In addition, Perl defines the following: Note that C<\w> matches a single alphanumeric character, not a whole word. To match a word you'd need to say C<\w+>. You may use C<\w>, -C<\W>, C<\s>, C<\S>, C<\d> and C<\D> within character classes (though not +C<\W>, C<\s>, C<\S>, C<\d>, and C<\D> within character classes (though not as either end of a range). Perl defines the following zero-width assertions: \b Match a word boundary \B Match a non-(word boundary) - \A Match only at beginning of string - \Z Match only at end of string (or before newline at the end) + \A Match at only beginning of string + \Z Match at only end of string (or before newline at the end) \G Match only where previous m//g left off A word boundary (C<\b>) is defined as a spot between two characters that @@ -175,13 +175,13 @@ outside the current pattern, this should not be relied upon. See the WARNING below.) The scope of $E<lt>digitE<gt> (and C<$`>, C<$&>, and C<$'>) extends to the end of the enclosing BLOCK or eval string, or to the next successful pattern match, whichever comes first. If you want to use -parentheses to delimit a subpattern (e.g. a set of alternatives) without +parentheses to delimit a subpattern (e.g., a set of alternatives) without saving it as a subpattern, follow the ( with a ?:. You may have as many parentheses as you wish. If you have more than 9 substrings, the variables $10, $11, ... refer to the corresponding substring. Within the pattern, \10, \11, etc. refer back -to substrings if there have been at least that many left parens before +to substrings if there have been at least that many left parentheses before the backreference. Otherwise (for backward compatibility) \10 is the same as \010, a backspace, and \11 the same as \011, a tab. And so on. (\1 through \9 are always backreferences.) @@ -203,9 +203,9 @@ You will note that all backslashed metacharacters in Perl are alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular expression languages, there are no backslashed symbols that aren't alphanumeric. So anything that looks like \\, \(, \), \E<lt>, \E<gt>, \{, or \} is always -interpreted as a literal character, not a metacharacter. This makes it +interpreted as a literal character, not a meta-character. This makes it simple to quote a string that you want to use for a pattern but that -you are afraid might contain metacharacters. Simply quote all the +you are afraid might contain metacharacters. Quote simply all the non-alphanumeric characters: $pattern =~ s/(\W)/\\$1/g; @@ -216,11 +216,11 @@ is to say /$unquoted\Q$quoted\E$unquoted/ -Perl 5 defines a consistent extension syntax for regular expressions. -The syntax is a pair of parens with a question mark as the first thing -within the parens (this was a syntax error in Perl 4). The character -after the question mark gives the function of the extension. Several -extensions are already supported: +Perl defines a consistent extension syntax for regular expressions. +The syntax is a pair of parentheses with a question mark as the first +thing within the parentheses (this was a syntax error in older +versions of Perl). The character after the question mark gives the +function of the extension. Several extensions are already supported: =over 10 @@ -268,7 +268,7 @@ easier just to say: One or more embedded pattern-match modifiers. This is particularly useful for patterns that are specified in a table somewhere, some of which want to be case sensitive, and some of which don't. The case -insensitive ones merely need to include C<(?i)> at the front of the +insensitive ones need to include merely C<(?i)> at the front of the pattern. For example: $pattern = "foobar"; @@ -390,11 +390,10 @@ As you see, this can be a bit tricky. It's important to realize that a regular expression is merely a set of assertions that gives a definition of success. There may be 0, 1, or several different ways that the definition might succeed against a particular string. And if there are -multiple ways it might succeed, you need to understand backtracking in -order to know which variety of success you will achieve. +multiple ways it might succeed, you need to understand backtracking to know which variety of success you will achieve. When using lookahead assertions and negations, this can all get even -tricker. Imagine you'd like to find a sequence of nondigits not +tricker. Imagine you'd like to find a sequence of non-digits not followed by "123". You might try to write that as $_ = "ABC123"; @@ -421,12 +420,12 @@ This prints 3: got AB 4: got ABC -You might have expected test 3 to fail because it just seems to a more +You might have expected test 3 to fail because it seems to a more general purpose version of test 1. The important difference between them is that test 3 contains a quantifier (C<\D*>) and so can use backtracking, whereas test 1 will not. What's happening is that you've asked "Is it true that at the start of $x, following 0 or more -nondigits, you have something that's not 123?" If the pattern matcher had +non-digits, you have something that's not 123?" If the pattern matcher had let C<\D*> expand to "ABC", this would have caused the whole pattern to fail. The search engine will initially match C<\D*> with "ABC". Then it will @@ -437,7 +436,7 @@ in the hope of matching the complete regular expression. Well now, the pattern really, I<really> wants to succeed, so it uses the -standard regexp backoff-and-retry and lets C<\D*> expand to just "AB" this +standard regexp back-off-and-retry and lets C<\D*> expand to just "AB" this time. Now there's indeed something following "AB" that is not "123". It's in fact "C123", which suffices. @@ -477,10 +476,10 @@ it would take literally forever--or until you ran out of stack space. In case you're not familiar with the "regular" Version 8 regexp routines, here are the pattern-matching rules not described above. -Any single character matches itself, unless it is a I<metacharacter> +Any single character matches itself, unless it is a I<meta-character> with a special meaning described here or above. You can cause characters which normally function as metacharacters to be interpreted -literally by prefixing them with a "\" (e.g. "\." matches a ".", not any +literally by prefixing them with a "\" (e.g., "\." matches a ".", not any character; "\\" matches a "\"). A series of characters matches that series of characters in the target string, so the pattern C<blurfl> would match "blurfl" in the target string. @@ -492,13 +491,13 @@ in the list. Within a list, the "-" character is used to specify a range, so that C<a-z> represents all the characters between "a" and "z", inclusive. -Characters may be specified using a metacharacter syntax much like that +Characters may be specified using a meta-character syntax much like that used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return, "\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string of octal digits, matches the character whose ASCII value is I<nnn>. Similarly, \xI<nn>, where I<nn> are hexadecimal digits, matches the character whose ASCII value is I<nn>. The expression \cI<x> matches the -ASCII character control-I<x>. Finally, the "." metacharacter matches any +ASCII character control-I<x>. Finally, the "." meta-character matches any character except "\n" (unless you use C</s>). You can specify a series of alternatives for a pattern using "|" to @@ -513,14 +512,14 @@ start and end. Note however that "|" is interpreted as a literal with square brackets, so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. -Within a pattern, you may designate subpatterns for later reference by +Within a pattern, you may designate sub-patterns for later reference by enclosing them in parentheses, and you may refer back to the I<n>th -subpattern later in the pattern using the metacharacter \I<n>. -Subpatterns are numbered based on the left to right order of their +sub-pattern later in the pattern using the meta-character \I<n>. +Sub-patterns are numbered based on the left to right order of their opening parenthesis. Note that a backreference matches whatever -actually matched the subpattern in the string being examined, not the -rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will -match "0x1234 0x4321",but not "0x1234 01234", since subpattern 1 +actually matched the sub-pattern in the string being examined, not the +rules for that sub-pattern. Therefore, C<(0|0x)\d*\s\1\d*> will +match "0x1234 0x4321",but not "0x1234 01234", because sub-pattern 1 actually matched "0x", even though the rule C<0|0x> could potentially match the leading 0 in the second number. @@ -532,7 +531,7 @@ Some people get too used to writing things like This is grandfathered for the RHS of a substitute to avoid shocking the B<sed> addicts, but it's a dirty habit to get into. That's because in -PerlThink, the right-hand side of a C<s///> is a double-quoted string. C<\1> in +PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in the usual double-quoted string means a control-A. The customary Unix meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit of doing that, you get yourself into trouble if you then add an C</e> |