Merge branch 'master' into feature/memory-work

4 files changed, 716 insertions, 536 deletions

diff --git a/doc/ChangeLog b/doc/ChangeLog
index 3dd82e08..34a0b68c 100644
--- a/doc/ChangeLog
+++ b/doc/ChangeLog
@@ -1,3 +1,10 @@
+2021-03-31         Arnold D. Robbins     <arnold@skeeve.com>
+
+	* gawktexi.in (Regexp Operator Details): Added a sidebar on
+	matching the empty regexp. Thanks to Arkadiusz Drabczyk
+	<arkadiusz@drabczyk.org> for pointing out the lack of
+	documentation.
+
 2021-03-21         Arnold D. Robbins     <arnold@skeeve.com>
 
 	* gawktexi.in (Other Environment Variables): Document "fnv1a"
diff --git a/doc/gawk.info b/doc/gawk.info
index 445d06dd..76ecc3c5 100644
--- a/doc/gawk.info
+++ b/doc/gawk.info
@@ -3911,6 +3911,39 @@ themselves when there is nothing in the regexp that precedes them.  For
 example, '/+/' matches a literal plus sign.  However, many other
 versions of 'awk' treat such a usage as a syntax error.
 
+                     What About The Empty Regexp?
+
+   We describe here an advanced regexp usage.  Feel free to skip it upon
+first reading.
+
+   You can supply an empty regexp constant ('//') in all places where a
+regexp is expected.  Is this useful?  What does it match?
+
+   It is useful.  It matches the (invisible) empty string at the start
+and end of a string of characters, as well as the empty string between
+characters.  This is best illustrated with the 'gsub()' function, which
+makes global substitutions in a string (*note String Functions::).
+Normal usage of 'gsub()' is like so:
+
+     $ awk '
+     > BEGIN {
+     >     x = "ABC_CBA"
+     >     gsub(/B/, "bb", x)
+     >     print x
+     > }'
+     -| AbbC_CbbA
+
+   We can use 'gsub()' to see where the empty strings are that match the
+empty regexp:
+
+     $ awk '
+     > BEGIN {
+     >     x = "ABC"
+     >     gsub(//, "x", x)
+     >     print x
+     > }'
+     -| xAxBxCx
+
    ---------- Footnotes ----------
 
    (1) In other literature, you may see a bracket expression referred to
@@ -35942,6 +35975,8 @@ Index
 * empty array elements:                  Reference to Elements.
                                                               (line  18)
 * empty pattern:                         Empty.               (line   6)
+* empty regexps:                         Regexp Operator Details.
+                                                              (line 149)
 * EMRED:                                 TCP/IP Networking.   (line   6)
 * enable breakpoint:                     Breakpoint Control.  (line  73)
 * enable debugger command:               Breakpoint Control.  (line  73)
@@ -37393,6 +37428,8 @@ Index
 * regexp constants <2>:                  Comparison Operators.
                                                               (line 103)
 * regexp constants, as patterns:         Expression Patterns. (line  34)
+* regexps, empty:                        Regexp Operator Details.
+                                                              (line 149)
 * register loadable extension:           Registration Functions.
                                                               (line   6)
 * regular expressions, interval expressions and: Options.     (line 317)
@@ -37604,6 +37641,8 @@ Index
                                                               (line 106)
 * sidebar, Escape Sequences for Metacharacters: Escape Sequences.
                                                               (line 138)
+* sidebar, What About The Empty Regexp?: Regexp Operator Details.
+                                                              (line 147)
 * sidebar, Using \n in Bracket Expressions of Dynamic Regexps: Computed Regexps.
                                                               (line  58)
 * sidebar, Caveats When Using Regular Expressions for RS: gawk split records.
@@ -38135,542 +38174,542 @@ Node: Regexp Usage165325
 Node: Escape Sequences167362
 Node: Regexp Operators173603
 Node: Regexp Operator Details174088
-Ref: Regexp Operator Details-Footnote-1180520
-Node: Interval Expressions180667
-Ref: Interval Expressions-Footnote-1182088
-Node: Bracket Expressions182186
-Ref: table-char-classes184662
-Node: Leftmost Longest187988
-Node: Computed Regexps189291
-Node: GNU Regexp Operators192718
-Node: Case-sensitivity196455
-Ref: Case-sensitivity-Footnote-1199321
-Ref: Case-sensitivity-Footnote-2199556
-Node: Regexp Summary199664
-Node: Reading Files201130
-Node: Records203399
-Node: awk split records204474
-Node: gawk split records209174
-Ref: gawk split records-Footnote-1214248
-Node: Fields214285
-Node: Nonconstant Fields217026
-Ref: Nonconstant Fields-Footnote-1219262
-Node: Changing Fields219466
-Node: Field Separators225497
-Node: Default Field Splitting228195
-Node: Regexp Field Splitting229313
-Node: Single Character Fields232990
-Node: Command Line Field Separator234050
-Node: Full Line Fields237268
-Ref: Full Line Fields-Footnote-1238790
-Ref: Full Line Fields-Footnote-2238836
-Node: Field Splitting Summary238937
-Node: Constant Size241011
-Node: Fixed width data241743
-Node: Skipping intervening245210
-Node: Allowing trailing data246008
-Node: Fields with fixed data247045
-Node: Splitting By Content248563
-Ref: Splitting By Content-Footnote-1252346
-Node: More CSV252509
-Node: Testing field creation254101
-Node: Multiple Line255726
-Node: Getline262003
-Node: Plain Getline264472
-Node: Getline/Variable267045
-Node: Getline/File268196
-Node: Getline/Variable/File269584
-Ref: Getline/Variable/File-Footnote-1271189
-Node: Getline/Pipe271277
-Node: Getline/Variable/Pipe273981
-Node: Getline/Coprocess275116
-Node: Getline/Variable/Coprocess276383
-Node: Getline Notes277125
-Node: Getline Summary279922
-Ref: table-getline-variants280346
-Node: Read Timeout281094
-Ref: Read Timeout-Footnote-1285000
-Node: Retrying Input285058
-Node: Command-line directories286257
-Node: Input Summary287163
-Node: Input Exercises290335
-Node: Printing290769
-Node: Print292603
-Node: Print Examples294060
-Node: Output Separators296840
-Node: OFMT298857
-Node: Printf300213
-Node: Basic Printf300998
-Node: Control Letters302572
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-Node: Special FD323076
-Ref: Special FD-Footnote-1326244
-Node: Special Files326318
-Node: Other Inherited Files326935
-Node: Special Network327936
-Node: Special Caveats328796
-Node: Close Files And Pipes329745
-Ref: table-close-pipe-return-values336652
-Ref: Close Files And Pipes-Footnote-1337465
-Ref: Close Files And Pipes-Footnote-2337613
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-Node: Constants343870
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-Ref: Scalar Constants-Footnote-1347071
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-Node: Using Constant Regexps350848
-Node: Standard Regexp Constants351470
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-Node: Variables357670
-Node: Using Variables358327
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-Ref: POSIX String Comparison-Footnote-1401006
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-Node: Cached values1089055
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-Node: PC Binary Installation1243057
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-Node: GNU Free Documentation License1387243
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+Ref: Regexp Operator Details-Footnote-1181452
+Node: Interval Expressions181599
+Ref: Interval Expressions-Footnote-1183020
+Node: Bracket Expressions183118
+Ref: table-char-classes185594
+Node: Leftmost Longest188920
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+Node: GNU Regexp Operators193650
+Node: Case-sensitivity197387
+Ref: Case-sensitivity-Footnote-1200253
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+Node: Regexp Summary200596
+Node: Reading Files202062
+Node: Records204331
+Node: awk split records205406
+Node: gawk split records210106
+Ref: gawk split records-Footnote-1215180
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+Node: Nonconstant Fields217958
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+Node: Full Line Fields238200
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+Node: Getline/Pipe272209
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+Ref: Auto-set-Footnote-2493959
+Node: ARGC and ARGV494015
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+Ref: figure-array-elements504806
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+Node: Reference to Elements507638
+Node: Assigning Elements510102
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+Node: Scanning an Array512352
+Node: Controlling Scanning515374
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+Node: Numeric Array Subscripts522146
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+Node: Multidimensional528758
+Node: Multiscanning531853
+Node: Arrays of Arrays533444
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+Node: Calling Built-in542424
+Node: Numeric Functions544420
+Ref: Numeric Functions-Footnote-1548446
+Ref: Numeric Functions-Footnote-2549094
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+Node: String Functions549414
+Ref: String Functions-Footnote-1573555
+Ref: String Functions-Footnote-2573683
+Ref: String Functions-Footnote-3573931
+Node: Gory Details574018
+Ref: table-sub-escapes575809
+Ref: table-sub-proposed577328
+Ref: table-posix-sub578691
+Ref: table-gensub-escapes580232
+Ref: Gory Details-Footnote-1581055
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+Ref: table-system-return-values587663
+Ref: I/O Functions-Footnote-1589743
+Ref: I/O Functions-Footnote-2589891
+Node: Time Functions590011
+Ref: Time Functions-Footnote-1600682
+Ref: Time Functions-Footnote-2600750
+Ref: Time Functions-Footnote-3600908
+Ref: Time Functions-Footnote-4601019
+Ref: Time Functions-Footnote-5601131
+Ref: Time Functions-Footnote-6601358
+Node: Bitwise Functions601624
+Ref: table-bitwise-ops602218
+Ref: Bitwise Functions-Footnote-1608281
+Ref: Bitwise Functions-Footnote-2608454
+Node: Type Functions608645
+Node: I18N Functions611508
+Node: User-defined613159
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+Ref: Function Example-Footnote-1622658
+Node: Function Calling622680
+Node: Calling A Function623268
+Node: Variable Scope624226
+Node: Pass By Value/Reference627220
+Node: Function Caveats629864
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+Node: Return Statement632031
+Node: Dynamic Typing635010
+Node: Indirect Calls635940
+Ref: Indirect Calls-Footnote-1646192
+Node: Functions Summary646320
+Node: Library Functions649025
+Ref: Library Functions-Footnote-1652632
+Ref: Library Functions-Footnote-2652775
+Node: Library Names652946
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+Ref: Library Names-Footnote-2656836
+Node: General Functions656922
+Node: Strtonum Function658025
+Node: Assert Function661047
+Node: Round Function664373
+Node: Cliff Random Function665913
+Node: Ordinal Functions666929
+Ref: Ordinal Functions-Footnote-1669992
+Ref: Ordinal Functions-Footnote-2670244
+Node: Join Function670454
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+Node: Getlocaltime Function672424
+Node: Readfile Function676166
+Node: Shell Quoting678143
+Node: Data File Management679544
+Node: Filetrans Function680176
+Node: Rewind Function684272
+Node: File Checking686181
+Ref: File Checking-Footnote-1687515
+Node: Empty Files687716
+Node: Ignoring Assigns689695
+Node: Getopt Function691245
+Ref: Getopt Function-Footnote-1706456
+Node: Passwd Functions706656
+Ref: Passwd Functions-Footnote-1715495
+Node: Group Functions715583
+Ref: Group Functions-Footnote-1723481
+Node: Walking Arrays723688
+Node: Library Functions Summary726696
+Node: Library Exercises728102
+Node: Sample Programs728567
+Node: Running Examples729337
+Node: Clones730065
+Node: Cut Program731289
+Node: Egrep Program741429
+Node: Id Program750430
+Node: Split Program760377
+Ref: Split Program-Footnote-1770267
+Node: Tee Program770440
+Node: Uniq Program773230
+Node: Wc Program780818
+Node: Bytes vs. Characters781215
+Node: Using extensions782763
+Node: wc program783517
+Node: Miscellaneous Programs788382
+Node: Dupword Program789595
+Node: Alarm Program791625
+Node: Translate Program796480
+Ref: Translate Program-Footnote-1801045
+Node: Labels Program801315
+Ref: Labels Program-Footnote-1804666
+Node: Word Sorting804750
+Node: History Sorting808822
+Node: Extract Program811047
+Node: Simple Sed819101
+Node: Igawk Program822175
+Ref: Igawk Program-Footnote-1836506
+Ref: Igawk Program-Footnote-2836708
+Ref: Igawk Program-Footnote-3836830
+Node: Anagram Program836945
+Node: Signature Program840007
+Node: Programs Summary841254
+Node: Programs Exercises842468
+Ref: Programs Exercises-Footnote-1846598
+Node: Advanced Features846684
+Node: Nondecimal Data848751
+Node: Array Sorting850342
+Node: Controlling Array Traversal851042
+Ref: Controlling Array Traversal-Footnote-1859410
+Node: Array Sorting Functions859528
+Ref: Array Sorting Functions-Footnote-1864619
+Node: Two-way I/O864815
+Ref: Two-way I/O-Footnote-1872536
+Ref: Two-way I/O-Footnote-2872723
+Node: TCP/IP Networking872805
+Node: Profiling875923
+Node: Extension Philosophy885232
+Node: Advanced Features Summary886711
+Node: Internationalization888726
+Node: I18N and L10N890206
+Node: Explaining gettext890893
+Ref: Explaining gettext-Footnote-1896785
+Ref: Explaining gettext-Footnote-2896970
+Node: Programmer i18n897135
+Ref: Programmer i18n-Footnote-1902084
+Node: Translator i18n902133
+Node: String Extraction902927
+Ref: String Extraction-Footnote-1904059
+Node: Printf Ordering904145
+Ref: Printf Ordering-Footnote-1906931
+Node: I18N Portability906995
+Ref: I18N Portability-Footnote-1909451
+Node: I18N Example909514
+Ref: I18N Example-Footnote-1912789
+Ref: I18N Example-Footnote-2912862
+Node: Gawk I18N912971
+Node: I18N Summary913620
+Node: Debugger914961
+Node: Debugging915961
+Node: Debugging Concepts916402
+Node: Debugging Terms918211
+Node: Awk Debugging920786
+Ref: Awk Debugging-Footnote-1921731
+Node: Sample Debugging Session921863
+Node: Debugger Invocation922397
+Node: Finding The Bug923783
+Node: List of Debugger Commands930257
+Node: Breakpoint Control931590
+Node: Debugger Execution Control935284
+Node: Viewing And Changing Data938646
+Node: Execution Stack942187
+Node: Debugger Info943824
+Node: Miscellaneous Debugger Commands947895
+Node: Readline Support952957
+Node: Limitations953853
+Node: Debugging Summary956407
+Node: Namespaces957686
+Node: Global Namespace958797
+Node: Qualified Names960195
+Node: Default Namespace961194
+Node: Changing The Namespace961935
+Node: Naming Rules963549
+Node: Internal Name Management965397
+Node: Namespace Example966439
+Node: Namespace And Features969001
+Node: Namespace Summary970436
+Node: Arbitrary Precision Arithmetic971913
+Node: Computer Arithmetic973400
+Ref: table-numeric-ranges977166
+Ref: table-floating-point-ranges977659
+Ref: Computer Arithmetic-Footnote-1978317
+Node: Math Definitions978374
+Ref: table-ieee-formats981350
+Node: MPFR features981917
+Node: FP Math Caution983635
+Ref: FP Math Caution-Footnote-1984707
+Node: Inexactness of computations985076
+Node: Inexact representation986107
+Node: Comparing FP Values987467
+Node: Errors accumulate988708
+Node: Strange values990164
+Ref: Strange values-Footnote-1992752
+Node: Getting Accuracy992857
+Node: Try To Round995567
+Node: Setting precision996466
+Ref: table-predefined-precision-strings997163
+Node: Setting the rounding mode998993
+Ref: table-gawk-rounding-modes999367
+Ref: Setting the rounding mode-Footnote-11003298
+Node: Arbitrary Precision Integers1003477
+Ref: Arbitrary Precision Integers-Footnote-11006652
+Node: Checking for MPFR1006801
+Node: POSIX Floating Point Problems1008275
+Ref: POSIX Floating Point Problems-Footnote-11012560
+Node: Floating point summary1012598
+Node: Dynamic Extensions1014788
+Node: Extension Intro1016341
+Node: Plugin License1017607
+Node: Extension Mechanism Outline1018404
+Ref: figure-load-extension1018843
+Ref: figure-register-new-function1020408
+Ref: figure-call-new-function1021500
+Node: Extension API Description1023562
+Node: Extension API Functions Introduction1025275
+Ref: table-api-std-headers1027111
+Node: General Data Types1031360
+Ref: General Data Types-Footnote-11039990
+Node: Memory Allocation Functions1040289
+Ref: Memory Allocation Functions-Footnote-11044790
+Node: Constructor Functions1044889
+Node: API Ownership of MPFR and GMP Values1048355
+Node: Registration Functions1049668
+Node: Extension Functions1050368
+Node: Exit Callback Functions1055690
+Node: Extension Version String1056940
+Node: Input Parsers1057603
+Node: Output Wrappers1070324
+Node: Two-way processors1074836
+Node: Printing Messages1077101
+Ref: Printing Messages-Footnote-11078272
+Node: Updating ERRNO1078425
+Node: Requesting Values1079164
+Ref: table-value-types-returned1079901
+Node: Accessing Parameters1080837
+Node: Symbol Table Access1082074
+Node: Symbol table by name1082586
+Ref: Symbol table by name-Footnote-11085610
+Node: Symbol table by cookie1085738
+Ref: Symbol table by cookie-Footnote-11089923
+Node: Cached values1089987
+Ref: Cached values-Footnote-11093523
+Node: Array Manipulation1093676
+Ref: Array Manipulation-Footnote-11094767
+Node: Array Data Types1094804
+Ref: Array Data Types-Footnote-11097462
+Node: Array Functions1097554
+Node: Flattening Arrays1102052
+Node: Creating Arrays1109028
+Node: Redirection API1113795
+Node: Extension API Variables1116628
+Node: Extension Versioning1117339
+Ref: gawk-api-version1117768
+Node: Extension GMP/MPFR Versioning1119499
+Node: Extension API Informational Variables1121127
+Node: Extension API Boilerplate1122200
+Node: Changes from API V11126174
+Node: Finding Extensions1127746
+Node: Extension Example1128305
+Node: Internal File Description1129103
+Node: Internal File Ops1133183
+Ref: Internal File Ops-Footnote-11144533
+Node: Using Internal File Ops1144673
+Ref: Using Internal File Ops-Footnote-11147056
+Node: Extension Samples1147330
+Node: Extension Sample File Functions1148859
+Node: Extension Sample Fnmatch1156508
+Node: Extension Sample Fork1157995
+Node: Extension Sample Inplace1159213
+Node: Extension Sample Ord1162839
+Node: Extension Sample Readdir1163675
+Ref: table-readdir-file-types1164564
+Node: Extension Sample Revout1165631
+Node: Extension Sample Rev2way1166220
+Node: Extension Sample Read write array1166960
+Node: Extension Sample Readfile1168902
+Node: Extension Sample Time1169997
+Node: Extension Sample API Tests1171749
+Node: gawkextlib1172241
+Node: Extension summary1175159
+Node: Extension Exercises1178861
+Node: Language History1180103
+Node: V7/SVR3.11181759
+Node: SVR41183911
+Node: POSIX1185345
+Node: BTL1186726
+Node: POSIX/GNU1187455
+Node: Feature History1193233
+Node: Common Extensions1209552
+Node: Ranges and Locales1210835
+Ref: Ranges and Locales-Footnote-11215451
+Ref: Ranges and Locales-Footnote-21215478
+Ref: Ranges and Locales-Footnote-31215713
+Node: Contributors1215936
+Node: History summary1221933
+Node: Installation1223313
+Node: Gawk Distribution1224257
+Node: Getting1224741
+Node: Extracting1225704
+Node: Distribution contents1227342
+Node: Unix Installation1233822
+Node: Quick Installation1234504
+Node: Shell Startup Files1236918
+Node: Additional Configuration Options1238007
+Node: Configuration Philosophy1240322
+Node: Non-Unix Installation1242691
+Node: PC Installation1243151
+Node: PC Binary Installation1243989
+Node: PC Compiling1244424
+Node: PC Using1245541
+Node: Cygwin1249094
+Node: MSYS1250318
+Node: VMS Installation1250920
+Node: VMS Compilation1251711
+Ref: VMS Compilation-Footnote-11252940
+Node: VMS Dynamic Extensions1252998
+Node: VMS Installation Details1254683
+Node: VMS Running1256936
+Node: VMS GNV1261215
+Node: VMS Old Gawk1261950
+Node: Bugs1262421
+Node: Bug address1263084
+Node: Usenet1266066
+Node: Maintainers1267070
+Node: Other Versions1268255
+Node: Installation summary1276120
+Node: Notes1277329
+Node: Compatibility Mode1278123
+Node: Additions1278905
+Node: Accessing The Source1279830
+Node: Adding Code1281267
+Node: New Ports1287486
+Node: Derived Files1291861
+Ref: Derived Files-Footnote-11297521
+Ref: Derived Files-Footnote-21297556
+Ref: Derived Files-Footnote-31298154
+Node: Future Extensions1298268
+Node: Implementation Limitations1298926
+Node: Extension Design1300136
+Node: Old Extension Problems1301280
+Ref: Old Extension Problems-Footnote-11302798
+Node: Extension New Mechanism Goals1302855
+Ref: Extension New Mechanism Goals-Footnote-11306219
+Node: Extension Other Design Decisions1306408
+Node: Extension Future Growth1308521
+Node: Notes summary1309127
+Node: Basic Concepts1310285
+Node: Basic High Level1310966
+Ref: figure-general-flow1311248
+Ref: figure-process-flow1311933
+Ref: Basic High Level-Footnote-11315234
+Node: Basic Data Typing1315419
+Node: Glossary1318747
+Node: Copying1350632
+Node: GNU Free Documentation License1388175
+Node: Index1413295
 
 End Tag Table
 
diff --git a/doc/gawk.texi b/doc/gawk.texi
index b472316b..591246c0 100644
--- a/doc/gawk.texi
+++ b/doc/gawk.texi
@@ -5970,6 +5970,100 @@ regexp that precedes them.  For example, @code{/+/} matches a literal
 plus sign.  However, many other versions of @command{awk} treat such a
 usage as a syntax error.
 
+@cindex sidebar @subentry What About The Empty Regexp?
+@ifdocbook
+@docbook
+<sidebar><title>What About The Empty Regexp?</title>
+@end docbook
+
+@cindex empty regexps
+@cindex regexps, empty
+We describe here an advanced regexp usage. Feel free to skip it
+upon first reading.
+
+You can supply an empty regexp constant (@samp{//}) in all places
+where a regexp is expected. Is this useful?  What does it match?
+
+It is useful. It matches the (invisible) empty string at the start
+and end of a string of characters, as well as the empty string
+between characters. This is best illustrated with the @code{gsub()}
+function, which makes global substitutions in a string
+(@pxref{String Functions}).  Normal usage of @code{gsub()} is like
+so:
+
+@example
+$ @kbd{awk '}
+> @kbd{BEGIN @{}
+> @kbd{    x = "ABC_CBA"}
+> @kbd{    gsub(/B/, "bb", x)}
+> @kbd{    print x}
+> @kbd{@}'}
+@print{} AbbC_CbbA
+@end example
+
+We can use @code{gsub()} to see where the empty strings
+are that match the empty regexp:
+
+@example
+$ @kbd{awk '}
+> @kbd{BEGIN @{}
+> @kbd{    x = "ABC"}
+> @kbd{    gsub(//, "x", x)}
+> @kbd{    print x}
+> @kbd{@}'}
+@print{} xAxBxCx
+@end example
+
+@docbook
+</sidebar>
+@end docbook
+@end ifdocbook
+
+@ifnotdocbook
+@cartouche
+@center @b{What About The Empty Regexp?}
+
+
+@cindex empty regexps
+@cindex regexps, empty
+We describe here an advanced regexp usage. Feel free to skip it
+upon first reading.
+
+You can supply an empty regexp constant (@samp{//}) in all places
+where a regexp is expected. Is this useful?  What does it match?
+
+It is useful. It matches the (invisible) empty string at the start
+and end of a string of characters, as well as the empty string
+between characters. This is best illustrated with the @code{gsub()}
+function, which makes global substitutions in a string
+(@pxref{String Functions}).  Normal usage of @code{gsub()} is like
+so:
+
+@example
+$ @kbd{awk '}
+> @kbd{BEGIN @{}
+> @kbd{    x = "ABC_CBA"}
+> @kbd{    gsub(/B/, "bb", x)}
+> @kbd{    print x}
+> @kbd{@}'}
+@print{} AbbC_CbbA
+@end example
+
+We can use @code{gsub()} to see where the empty strings
+are that match the empty regexp:
+
+@example
+$ @kbd{awk '}
+> @kbd{BEGIN @{}
+> @kbd{    x = "ABC"}
+> @kbd{    gsub(//, "x", x)}
+> @kbd{    print x}
+> @kbd{@}'}
+@print{} xAxBxCx
+@end example
+@end cartouche
+@end ifnotdocbook
+
 @node Interval Expressions
 @subsection Some Notes On Interval Expressions
 
diff --git a/doc/gawktexi.in b/doc/gawktexi.in
index 2c2d7297..38e9dc0e 100644
--- a/doc/gawktexi.in
+++ b/doc/gawktexi.in
@@ -5752,6 +5752,46 @@ regexp that precedes them.  For example, @code{/+/} matches a literal
 plus sign.  However, many other versions of @command{awk} treat such a
 usage as a syntax error.
 
+@sidebar What About The Empty Regexp?
+@cindex empty regexps
+@cindex regexps, empty
+We describe here an advanced regexp usage. Feel free to skip it
+upon first reading.
+
+You can supply an empty regexp constant (@samp{//}) in all places
+where a regexp is expected. Is this useful?  What does it match?
+
+It is useful. It matches the (invisible) empty string at the start
+and end of a string of characters, as well as the empty string
+between characters. This is best illustrated with the @code{gsub()}
+function, which makes global substitutions in a string
+(@pxref{String Functions}).  Normal usage of @code{gsub()} is like
+so:
+
+@example
+$ @kbd{awk '}
+> @kbd{BEGIN @{}
+> @kbd{    x = "ABC_CBA"}
+> @kbd{    gsub(/B/, "bb", x)}
+> @kbd{    print x}
+> @kbd{@}'}
+@print{} AbbC_CbbA
+@end example
+
+We can use @code{gsub()} to see where the empty strings
+are that match the empty regexp:
+
+@example
+$ @kbd{awk '}
+> @kbd{BEGIN @{}
+> @kbd{    x = "ABC"}
+> @kbd{    gsub(//, "x", x)}
+> @kbd{    print x}
+> @kbd{@}'}
+@print{} xAxBxCx
+@end example
+@end sidebar
+
 @node Interval Expressions
 @subsection Some Notes On Interval Expressions