Re: regexp curly braces do not work

[Eric Blake]

On 7/30/20 1:24 AM, Hyunho Cho wrote:
> m4 (GNU M4) 1.4.18
> Operating System: Ubuntu 20.04 LTS
> Kernel: Linux 5.4.0-33-generic
> Architecture: x86-64
>
>
> sh$ m4 <<\@
> patsubst(`hello', `ll', `XX')
> patsubst(`hello', `l+', `XX')
> patsubst(`hello', `l\{2\}', `XX')
> patsubst(`hello', `l{2}', `XX')
> @
> heXXo
> heXXo
> hello
> hello

Unfortunately correct for m4 1.4.  The source code uses GNU regex.h, and 
does not bother to modify re_syntax or call re_set_syntax(), which means 
m4 defaults to RE_SYNTAX_EMACS (ie. all regex bits set to 0).  Looking 
at the various bits that can be set for re_syntax,

/* If this bit is set, either \{...\} or {...} defines an
     interval, depending on RE_NO_BK_BRACES.
   If not set, \{, \}, {, and } are literals.  */
# define RE_INTERVALS (RE_HAT_LISTS_NOT_NEWLINE << 1)

and since m4 isn't doing anything about that bit, no form of {} works.

At some point, a future m4 release will add the ability to set regex 
flavors (emacs, POSIX BRE, POSIX ERE, etc), and depending on the flavor 
chosen, either {} or \{\} will work.  But until that release happens, 
you are stuck with the existing m4 being limited in expressive power.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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