Add some doc on the empty regexp.

4 files changed, 715 insertions, 535 deletions

diff --git a/doc/ChangeLog b/doc/ChangeLog
index 77d280bd..04fff857 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.
+
 2021-03-18         Arnold D. Robbins     <arnold@skeeve.com>
 
 	* texinfo.tex: Updated from GNULIB.
diff --git a/doc/gawk.info b/doc/gawk.info
index d29e3525..4f0eef36 100644
--- a/doc/gawk.info
+++ b/doc/gawk.info
@@ -3908,6 +3908,39 @@ 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
@@ -35893,6 +35926,8 @@ 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)
@@ -37344,6 +37379,8 @@ 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)
@@ -37555,6 +37592,8 @@ 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.
@@ -38086,541 +38125,541 @@ Node: Regexp Usage165115
 Node: Escape Sequences167152
 Node: Regexp Operators173393
 Node: Regexp Operator Details173878
-Ref: Regexp Operator Details-Footnote-1180310
-Node: Interval Expressions180457
-Ref: Interval Expressions-Footnote-1181878
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-Ref: table-char-classes184452
-Node: Leftmost Longest187778
-Node: Computed Regexps189081
-Node: GNU Regexp Operators192508
-Node: Case-sensitivity196245
-Ref: Case-sensitivity-Footnote-1199111
-Ref: Case-sensitivity-Footnote-2199346
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-Node: awk split records204264
-Node: gawk split records208964
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-Ref: Nonconstant Fields-Footnote-1219052
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-Ref: Full Line Fields-Footnote-2238626
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-Node: Fixed width data241533
-Node: Skipping intervening245000
-Node: Allowing trailing data245798
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-Ref: Splitting By Content-Footnote-1252136
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+Node: ARGC and ARGV493807
+Node: Pattern Action Summary498020
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+Ref: figure-array-elements504598
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+Node: Assigning Elements509894
+Node: Array Example510385
+Node: Scanning an Array512144
+Node: Controlling Scanning515166
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+Node: Multiscanning531645
+Node: Arrays of Arrays533236
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+Node: Built-in541135
+Node: Calling Built-in542216
+Node: Numeric Functions544212
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+Node: String Functions549208
+Ref: String Functions-Footnote-1573349
+Ref: String Functions-Footnote-2573477
+Ref: String Functions-Footnote-3573725
+Node: Gory Details573812
+Ref: table-sub-escapes575603
+Ref: table-sub-proposed577122
+Ref: table-posix-sub578485
+Ref: table-gensub-escapes580026
+Ref: Gory Details-Footnote-1580849
+Node: I/O Functions581003
+Ref: table-system-return-values587457
+Ref: I/O Functions-Footnote-1589537
+Ref: I/O Functions-Footnote-2589685
+Node: Time Functions589805
+Ref: Time Functions-Footnote-1600476
+Ref: Time Functions-Footnote-2600544
+Ref: Time Functions-Footnote-3600702
+Ref: Time Functions-Footnote-4600813
+Ref: Time Functions-Footnote-5600925
+Ref: Time Functions-Footnote-6601152
+Node: Bitwise Functions601418
+Ref: table-bitwise-ops602012
+Ref: Bitwise Functions-Footnote-1608075
+Ref: Bitwise Functions-Footnote-2608248
+Node: Type Functions608439
+Node: I18N Functions611302
+Node: User-defined612953
+Node: Definition Syntax613765
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+Node: Function Example619530
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+Node: Function Calling622474
+Node: Calling A Function623062
+Node: Variable Scope624020
+Node: Pass By Value/Reference627014
+Node: Function Caveats629658
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+Node: Return Statement631825
+Node: Dynamic Typing634804
+Node: Indirect Calls635734
+Ref: Indirect Calls-Footnote-1645986
+Node: Functions Summary646114
+Node: Library Functions648819
+Ref: Library Functions-Footnote-1652426
+Ref: Library Functions-Footnote-2652569
+Node: Library Names652740
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+Ref: Library Names-Footnote-2656630
+Node: General Functions656716
+Node: Strtonum Function657819
+Node: Assert Function660841
+Node: Round Function664167
+Node: Cliff Random Function665707
+Node: Ordinal Functions666723
+Ref: Ordinal Functions-Footnote-1669786
+Ref: Ordinal Functions-Footnote-2670038
+Node: Join Function670248
+Ref: Join Function-Footnote-1672018
+Node: Getlocaltime Function672218
+Node: Readfile Function675960
+Node: Shell Quoting677937
+Node: Data File Management679338
+Node: Filetrans Function679970
+Node: Rewind Function684066
+Node: File Checking685975
+Ref: File Checking-Footnote-1687309
+Node: Empty Files687510
+Node: Ignoring Assigns689489
+Node: Getopt Function691039
+Ref: Getopt Function-Footnote-1706250
+Node: Passwd Functions706450
+Ref: Passwd Functions-Footnote-1715289
+Node: Group Functions715377
+Ref: Group Functions-Footnote-1723275
+Node: Walking Arrays723482
+Node: Library Functions Summary726490
+Node: Library Exercises727896
+Node: Sample Programs728361
+Node: Running Examples729131
+Node: Clones729859
+Node: Cut Program731083
+Node: Egrep Program741223
+Node: Id Program750224
+Node: Split Program760171
+Ref: Split Program-Footnote-1770061
+Node: Tee Program770234
+Node: Uniq Program773024
+Node: Wc Program780612
+Node: Bytes vs. Characters781009
+Node: Using extensions782557
+Node: wc program783311
+Node: Miscellaneous Programs788176
+Node: Dupword Program789389
+Node: Alarm Program791419
+Node: Translate Program796274
+Ref: Translate Program-Footnote-1800839
+Node: Labels Program801109
+Ref: Labels Program-Footnote-1804460
+Node: Word Sorting804544
+Node: History Sorting808616
+Node: Extract Program810841
+Node: Simple Sed818895
+Node: Igawk Program821969
+Ref: Igawk Program-Footnote-1836300
+Ref: Igawk Program-Footnote-2836502
+Ref: Igawk Program-Footnote-3836624
+Node: Anagram Program836739
+Node: Signature Program839801
+Node: Programs Summary841048
+Node: Programs Exercises842262
+Ref: Programs Exercises-Footnote-1846392
+Node: Advanced Features846478
+Node: Nondecimal Data848545
+Node: Array Sorting850136
+Node: Controlling Array Traversal850836
+Ref: Controlling Array Traversal-Footnote-1859204
+Node: Array Sorting Functions859322
+Ref: Array Sorting Functions-Footnote-1864413
+Node: Two-way I/O864609
+Ref: Two-way I/O-Footnote-1872330
+Ref: Two-way I/O-Footnote-2872517
+Node: TCP/IP Networking872599
+Node: Profiling875717
+Node: Extension Philosophy885026
+Node: Advanced Features Summary886505
+Node: Internationalization888520
+Node: I18N and L10N890000
+Node: Explaining gettext890687
+Ref: Explaining gettext-Footnote-1896579
+Ref: Explaining gettext-Footnote-2896764
+Node: Programmer i18n896929
+Ref: Programmer i18n-Footnote-1901878
+Node: Translator i18n901927
+Node: String Extraction902721
+Ref: String Extraction-Footnote-1903853
+Node: Printf Ordering903939
+Ref: Printf Ordering-Footnote-1906725
+Node: I18N Portability906789
+Ref: I18N Portability-Footnote-1909245
+Node: I18N Example909308
+Ref: I18N Example-Footnote-1912583
+Ref: I18N Example-Footnote-2912656
+Node: Gawk I18N912765
+Node: I18N Summary913414
+Node: Debugger914755
+Node: Debugging915755
+Node: Debugging Concepts916196
+Node: Debugging Terms918005
+Node: Awk Debugging920580
+Ref: Awk Debugging-Footnote-1921525
+Node: Sample Debugging Session921657
+Node: Debugger Invocation922191
+Node: Finding The Bug923577
+Node: List of Debugger Commands930051
+Node: Breakpoint Control931384
+Node: Debugger Execution Control935078
+Node: Viewing And Changing Data938440
+Node: Execution Stack941981
+Node: Debugger Info943618
+Node: Miscellaneous Debugger Commands947689
+Node: Readline Support952751
+Node: Limitations953647
+Node: Debugging Summary956201
+Node: Namespaces957480
+Node: Global Namespace958591
+Node: Qualified Names959989
+Node: Default Namespace960988
+Node: Changing The Namespace961729
+Node: Naming Rules963343
+Node: Internal Name Management965191
+Node: Namespace Example966233
+Node: Namespace And Features968795
+Node: Namespace Summary970230
+Node: Arbitrary Precision Arithmetic971707
+Node: Computer Arithmetic973194
+Ref: table-numeric-ranges976960
+Ref: table-floating-point-ranges977453
+Ref: Computer Arithmetic-Footnote-1978111
+Node: Math Definitions978168
+Ref: table-ieee-formats981484
+Ref: Math Definitions-Footnote-1982087
+Node: MPFR features982192
+Node: FP Math Caution983910
+Ref: FP Math Caution-Footnote-1984982
+Node: Inexactness of computations985351
+Node: Inexact representation986311
+Node: Comparing FP Values987671
+Node: Errors accumulate988912
+Node: Getting Accuracy990345
+Node: Try To Round993055
+Node: Setting precision993954
+Ref: table-predefined-precision-strings994651
+Node: Setting the rounding mode996481
+Ref: table-gawk-rounding-modes996855
+Ref: Setting the rounding mode-Footnote-11000786
+Node: Arbitrary Precision Integers1000965
+Ref: Arbitrary Precision Integers-Footnote-11004140
+Node: Checking for MPFR1004289
+Node: POSIX Floating Point Problems1005763
+Ref: POSIX Floating Point Problems-Footnote-11010048
+Node: Floating point summary1010086
+Node: Dynamic Extensions1012276
+Node: Extension Intro1013829
+Node: Plugin License1015095
+Node: Extension Mechanism Outline1015892
+Ref: figure-load-extension1016331
+Ref: figure-register-new-function1017896
+Ref: figure-call-new-function1018988
+Node: Extension API Description1021050
+Node: Extension API Functions Introduction1022763
+Ref: table-api-std-headers1024599
+Node: General Data Types1028848
+Ref: General Data Types-Footnote-11037478
+Node: Memory Allocation Functions1037777
+Ref: Memory Allocation Functions-Footnote-11042278
+Node: Constructor Functions1042377
+Node: API Ownership of MPFR and GMP Values1045843
+Node: Registration Functions1047156
+Node: Extension Functions1047856
+Node: Exit Callback Functions1053178
+Node: Extension Version String1054428
+Node: Input Parsers1055091
+Node: Output Wrappers1067812
+Node: Two-way processors1072324
+Node: Printing Messages1074589
+Ref: Printing Messages-Footnote-11075760
+Node: Updating ERRNO1075913
+Node: Requesting Values1076652
+Ref: table-value-types-returned1077389
+Node: Accessing Parameters1078325
+Node: Symbol Table Access1079562
+Node: Symbol table by name1080074
+Ref: Symbol table by name-Footnote-11083098
+Node: Symbol table by cookie1083226
+Ref: Symbol table by cookie-Footnote-11087411
+Node: Cached values1087475
+Ref: Cached values-Footnote-11091011
+Node: Array Manipulation1091164
+Ref: Array Manipulation-Footnote-11092255
+Node: Array Data Types1092292
+Ref: Array Data Types-Footnote-11094950
+Node: Array Functions1095042
+Node: Flattening Arrays1099540
+Node: Creating Arrays1106516
+Node: Redirection API1111283
+Node: Extension API Variables1114116
+Node: Extension Versioning1114827
+Ref: gawk-api-version1115256
+Node: Extension GMP/MPFR Versioning1116987
+Node: Extension API Informational Variables1118615
+Node: Extension API Boilerplate1119688
+Node: Changes from API V11123662
+Node: Finding Extensions1125234
+Node: Extension Example1125793
+Node: Internal File Description1126591
+Node: Internal File Ops1130671
+Ref: Internal File Ops-Footnote-11142021
+Node: Using Internal File Ops1142161
+Ref: Using Internal File Ops-Footnote-11144544
+Node: Extension Samples1144818
+Node: Extension Sample File Functions1146347
+Node: Extension Sample Fnmatch1153996
+Node: Extension Sample Fork1155483
+Node: Extension Sample Inplace1156701
+Node: Extension Sample Ord1160327
+Node: Extension Sample Readdir1161163
+Ref: table-readdir-file-types1162052
+Node: Extension Sample Revout1163119
+Node: Extension Sample Rev2way1163708
+Node: Extension Sample Read write array1164448
+Node: Extension Sample Readfile1166390
+Node: Extension Sample Time1167485
+Node: Extension Sample API Tests1169237
+Node: gawkextlib1169729
+Node: Extension summary1172647
+Node: Extension Exercises1176349
+Node: Language History1177591
+Node: V7/SVR3.11179247
+Node: SVR41181399
+Node: POSIX1182833
+Node: BTL1184214
+Node: POSIX/GNU1184943
+Node: Feature History1190721
+Node: Common Extensions1207040
+Node: Ranges and Locales1208323
+Ref: Ranges and Locales-Footnote-11212939
+Ref: Ranges and Locales-Footnote-21212966
+Ref: Ranges and Locales-Footnote-31213201
+Node: Contributors1213424
+Node: History summary1219421
+Node: Installation1220801
+Node: Gawk Distribution1221745
+Node: Getting1222229
+Node: Extracting1223192
+Node: Distribution contents1224830
+Node: Unix Installation1231310
+Node: Quick Installation1231992
+Node: Shell Startup Files1234406
+Node: Additional Configuration Options1235495
+Node: Configuration Philosophy1237810
+Node: Non-Unix Installation1240179
+Node: PC Installation1240639
+Node: PC Binary Installation1241477
+Node: PC Compiling1241912
+Node: PC Using1243029
+Node: Cygwin1246582
+Node: MSYS1247806
+Node: VMS Installation1248408
+Node: VMS Compilation1249199
+Ref: VMS Compilation-Footnote-11250428
+Node: VMS Dynamic Extensions1250486
+Node: VMS Installation Details1252171
+Node: VMS Running1254424
+Node: VMS GNV1258703
+Node: VMS Old Gawk1259438
+Node: Bugs1259909
+Node: Bug address1260572
+Node: Usenet1263554
+Node: Maintainers1264558
+Node: Other Versions1265743
+Node: Installation summary1273608
+Node: Notes1274817
+Node: Compatibility Mode1275611
+Node: Additions1276393
+Node: Accessing The Source1277318
+Node: Adding Code1278755
+Node: New Ports1284974
+Node: Derived Files1289349
+Ref: Derived Files-Footnote-11295009
+Ref: Derived Files-Footnote-21295044
+Ref: Derived Files-Footnote-31295642
+Node: Future Extensions1295756
+Node: Implementation Limitations1296414
+Node: Extension Design1297624
+Node: Old Extension Problems1298768
+Ref: Old Extension Problems-Footnote-11300286
+Node: Extension New Mechanism Goals1300343
+Ref: Extension New Mechanism Goals-Footnote-11303707
+Node: Extension Other Design Decisions1303896
+Node: Extension Future Growth1306009
+Node: Notes summary1306615
+Node: Basic Concepts1307773
+Node: Basic High Level1308454
+Ref: figure-general-flow1308736
+Ref: figure-process-flow1309421
+Ref: Basic High Level-Footnote-11312722
+Node: Basic Data Typing1312907
+Node: Glossary1316235
+Node: Copying1348120
+Node: GNU Free Documentation License1385663
+Node: Index1410783
 
 End Tag Table
 
diff --git a/doc/gawk.texi b/doc/gawk.texi
index c208ecac..7a746e1b 100644
--- a/doc/gawk.texi
+++ b/doc/gawk.texi
@@ -5965,6 +5965,100 @@ 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 6ad9d6e0..a8c1cf86 100644
--- a/doc/gawktexi.in
+++ b/doc/gawktexi.in
@@ -5747,6 +5747,46 @@ 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