From victor@drawall.cc

Created by @Grimy

In a regex conditional expression (?(condition)yes-pattern|no-pattern), if the
condition is (?!), it fails the match outright instead of matching no-pattern.

This may be linked to the optimization documented later in perlre: “(?!) gets
optimised into (*FAIL) internally”. Doing this replacement when (?!) is the
condition of a conditional expression is invalid.

How to reproduce
----------------

perl5.23.4 -e 'print /(?(?!)(?{print "Yes"})|(?{print "No"}))/'

Expected behavior
-----------------

No1

Actual behavior
---------------

Perl prints nothing, indicating that:
1) Neither the yes-pattern nor the no-pattern were attempted;
2) The match failed, when it should have suceeded.

Affected versions
-----------------

Current blead (94757bf) is affected.
perl5.20.2 is affected.

Perl Info


Flags:
    category=core
    severity=low

Site configuration information for perl 5.23.4:

Configured by grimy at Wed Sep 30 09:47:59 CEST 2015.

Summary of my perl5 (revision 5 version 23 subversion 4) configuration:
  Commit id: 94757bf7ac8451b3588578f287f9c511a04011bf
  Platform:
    osname=linux, osvers=4.1.3-201.fc22.x86_64, archname=x86_64-linux
    uname='linux amaranth 4.1.3-201.fc22.x86_64 #1 smp wed jul 29
19:50:22 utc 2015 x86_64 x86_64 x86_64 gnulinux '
    config_args='-ds -e -Dusedevel'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=undef, usemultiplicity=undef
    use64bitint=define, use64bitall=define, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-fwrapv -fno-strict-aliasing -pipe
-fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE
-D_FILE_OFFSET_BITS=64',
    optimize='-O2',
    cppflags='-fwrapv -fno-strict-aliasing -pipe
-fstack-protector-strong -I/usr/local/include'
    ccversion='', gccversion='5.1.1 20150618 (Red Hat 5.1.1-4)', gccosandvers=''
    intsize=4, longsize=8, ptrsize=8, doublesize=8,
byteorder=12345678, doublekind=3
    d_longlong=define, longlongsize=8, d_longdbl=define,
longdblsize=16, longdblkind=3
    ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t',
lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib /lib/../lib64 /usr/lib/../lib64
/lib /lib64 /usr/lib64 /usr/local/lib64
    libs=-lpthread -lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc -lgdbm_compat
    perllibs=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc
    libc=libc-2.21.so, so=so, useshrplib=false, libperl=libperl.a
    gnulibc_version='2.21'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
    cccdlflags='-fPIC', lddlflags='-shared -O2 -L/usr/local/lib
-fstack-protector-strong'



@INC for perl 5.23.4:
    /usr/local/lib/perl5/site_perl/5.23.4/x86_64-linux
    /usr/local/lib/perl5/site_perl/5.23.4
    /usr/local/lib/perl5/5.23.4/x86_64-linux
    /usr/local/lib/perl5/5.23.4
    .


Environment for perl 5.23.4:
    HOME=/home/grimy
    LANG=en_US.UTF-8
    LANGUAGE (unset)
    LD_LIBRARY_PATH=/usr/lib64:/home/grimy/repos/SDK/InstallationCentOS59/IGC/Bin:/home/grimy/repos/SDK/Produits/QT-FC5/lib
    LOGDIR (unset)
    PATH=/home/grimy/bin:/home/grimy/.nvim/scripts:/home/grimy/.autojump/bin:/usr/java/jdk1.8.0_31/bin:/home/grimy/repos/CoreTex/bin:/home/grimy/bin:/home/grimy/.nvim/scripts:/home/grimy/.autojump/bin:/usr/java/jdk1.8.0_31/bin:/home/grimy/repos/CoreTex/bin:/usr/local/bin:/bin:/usr/bin:/home/grimy/repos/SDK/InstallationCentOS59/IGC/Bin:/home/grimy/bin:/home/grimy/.gem/ruby/2.0.0/bin:/sbin:/home/grimy/repos/SDK/InstallationCentOS59/IGC/Bin:/home/grimy/bin:/home/grimy/.gem/ruby/2.0.0/bin:/sbin
    PERL_BADLANG (unset)
    SHELL=/usr/bin/fish

regex: /(?(?!))/ incorrectly fails the match #14947

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