[perl #116537] Regex: (*THEN) doesn't work as described

# New Ticket Created by  Philip Hazel 
# Please include the string:  [perl #116537]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org:443/rt3/Ticket/Display.html?id=116537 >


Hello,

My understanding of how (*THEN) works is that the test below should 
match. The perlre page says "...this verb always matches, and when
backtracked into on failure, it causes the regex engine to try the next
alternation in the innermost enclosing group (capturing or otherwise)
that has alternations." Unless I am going mad, the examples below (one a 
normal group, the other an assertion) fulfil the condition.

$ perl -e 'print (("ac" =~ /^(?=ab|ac)/)? "yes\n":"no\n")'
yes
$ perl -e 'print (("ac" =~ /^(?=a(*THEN)b|ac)/)? "yes\n":"no\n")'
no

$ perl -e 'print (("ac" =~ /^(ab|ac)/)? "yes\n":"no\n")'
yes
$ perl -e 'print (("ac" =~ /^(a(*THEN)b|ac)/)? "yes\n":"no\n")'
no

I discovered this because PCRE does match these patterns. 

Regards,
Philip

-- 
Philip Hazel


---
Flags:
    category=core
    severity=low
---
Site configuration information for perl 5.16.2:

Configured by nobody at Tue Dec 11 22:56:31 CET 2012.

Summary of my perl5 (revision 5 version 16 subversion 2) configuration:
   
  Platform:
    osname=linux, osvers=3.6.9-1-arch, archname=i686-linux-thread-multi
    uname='linux flo 3.6.9-1-arch #1 smp preempt tue dec 4 08:04:10 cet 2012 i686 gnulinux '
    config_args='-des -Dusethreads -Duseshrplib -Doptimize=-march=i686 -mtune=generic -O2 -pipe -fstack-protector --param=ssp-buffer-size=4 -D_FORTIFY_SOURCE=2 -Dprefix=/usr -Dinstallprefix=/usr -Dvendorprefix=/usr -Dprivlib=/usr/share/perl5/core_perl -Darchlib=/usr/lib/perl5/core_perl -Dsitelib=/usr/share/perl5/site_perl -Dsitearch=/usr/lib/perl5/site_perl -Dvendorlib=/usr/share/perl5/vendor_perl -Dvendorarch=/usr/lib/perl5/vendor_perl -Dscriptdir=/usr/bin/core_perl -Dsitescript=/usr/bin/site_perl -Dvendorscript=/usr/bin/vendor_perl -Dinc_version_list=none -Dman1ext=1perl -Dman3ext=3perl -Dlddlflags=-shared -Wl,-O1,--sort-common,--as-needed,-z,relro -Dldflags=-Wl,-O1,--sort-common,--as-needed,-z,relro'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
    use64bitint=undef, use64bitall=undef, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-march=i686 -mtune=generic -O2 -pipe -fstack-protector --param=ssp-buffer-size=4 -D_FORTIFY_SOURCE=2',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
    ccversion='', gccversion='4.7.2', gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=4, prototype=define
  Linker and Libraries:
    ld='cc', ldflags ='-Wl,-O1,--sort-common,--as-needed,-z,relro -fstack-protector -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib
    libs=-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lpthread -lc -lgdbm_compat
    perllibs=-lnsl -ldl -lm -lcrypt -lutil -lpthread -lc
    libc=/lib/libc-2.16.so, so=so, useshrplib=true, libperl=libperl.so
    gnulibc_version='2.16'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E -Wl,-rpath,/usr/lib/perl5/core_perl/CORE'
    cccdlflags='-fPIC', lddlflags='-shared -Wl,-O1,--sort-common,--as-needed,-z,relro -L/usr/local/lib -fstack-protector'

Locally applied patches:
    

---
@INC for perl 5.16.2:
    /usr/lib/perl5/site_perl
    /usr/share/perl5/site_perl
    /usr/lib/perl5/vendor_perl
    /usr/share/perl5/vendor_perl
    /usr/lib/perl5/core_perl
    /usr/share/perl5/core_perl
    .

---
Environment for perl 5.16.2:
    HOME=/home/ph10
    LANG=en_GB
    LANGUAGE (unset)
    LC_ALL=C
    LC_COLLATE=C
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/ph10/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin:/sbin:/usr/bin/vendor_perl:/usr/bin/core_perl:.
    PERL_BADLANG (unset)
    SHELL=/bin/bash

• [perl #116537] Regex: (*THEN) doesn't work as described by Philip Hazel
• Re: [perl #116537] Regex: (*THEN) doesn't work as described by Ronald J Kimball
• Re: [perl #116537] Regex: (*THEN) doesn't work as described by Karl Williamson
• Re: [perl #116537] Regex: (*THEN) doesn't work as described by Dave Mitchell
• Re: [perl #116537] Regex: (*THEN) doesn't work as described by Brad Gilbert