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
Virtualization: qemu.org | libvirt.org
