Sotware versions
================
$ rpm -q m4
m4-1.4.18-9.fc29.x86_64
$ m4 --version
m4 (GNU M4) 1.4.18
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
<http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Written by Rene' Seindal.
Problem
=======
Lack of documentation on regular expressions.
m4 manual (<https://www.gnu.org/software/m4/manual/m4.html>) is very laconic
about regular expressions:
> Builtin: regexp (string, regexp, [replacement])
>
> Searches for regexp in string. The syntax for regular expressions is
> the same as in GNU Emacs, which is similar to BRE, Basic Regular
> Expressions in POSIX. See Syntax of Regular Expressions in the GNU
> Emacs Manual. Support for ERE, Extended Regular Expressions is not
> available, but will be added in GNU M4 2.0.
Ok, let us loot at GNU Emacs manual
(<https://www.gnu.org/software/emacs/manual/html_mono/emacs.html>). It
dpcuments \{n\} construct:
> 15.6 Syntax of Regular Expressions
> <...>
> \{n\}
> is a postfix operator specifying n repetitions—that is, the
> preceding regular expression must match exactly n times in a row.
> For example, ‘x\{4\}’ matches the string ‘xxxx’ and nothing else.
>
This construct is also documented in chapter "9.3.6 BREs Matching Multiple
Characters" of "The Open Group Base Specifications Issue 7, 2018 edition"
(<https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html>):
> 5. When a BRE matching a single character, a subexpression, or a
> back-reference is followed by an interval expression of the format
> "\{m\}", "\{m,\}", or "\{m,n\}", together with that interval expression
> it shall match what repeated consecutive occurrences of the BRE would
> match.
However, m4 does not recognize \{n\} construct. m4 just treats it literally:
$ cat input.txt
regexp(`aaaaaaa', `a\{3\}') # Not matched.
regexp(`a{3} ', `a\{3\}') # Matched.
$ m4 input.txt
-1 # Not matched.
0 # Matched.
It was very confusing and surprising. m4 manual must say a word that \{n\} is
not supported (as well as \{n,m\}).
