s// /g weirdness

[p5pRT]

Migrated from rt.perl.org#5959 (status was 'resolved')

Searchable as RT5959$

[p5pRT]

From @simoncozens

Maybe this is something obvious I missed, and I know I'm dealing with
somewhat undefined semantics, but...​:

  $_ = "this/is/a/test"; y#/#\n#; s// /g; print

prints

t h i s
i s
a
t e s t

as I'd expect, whereas

  $_ = "this/is/a/test"; s#/#\n#g; s// /g; print

prints

this
is
a
test

which I wouldn't. s//foo/ wants to be the first match in the program
or it doesn't match.

Perl Info

Flags:
    category=core
    severity=low

Site configuration information for perl v5.6.0:

Configured by bod at Tue Feb 27 03:27:16 EST 2001.

Summary of my perl5 (revision 5.0 version 6 subversion 0) configuration:
  Platform:
    osname=linux, osvers=2.2.18, archname=i386-linux
    uname='linux duende 2.2.18 #1 thu dec 28 14:51:40 est 2000 i686 unknown '
    config_args='-Dccflags=-DDEBIAN -Darchname=i386-linux -Dprefix=/usr -Dprivlib=/usr/share/perl/5.6.0 -Darchlib=/usr/lib/perl/5.6.0 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/perl5 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.6.0 -Dsitearch=/usr/local/lib/perl/5.6.0 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dman1ext=1 -Dman3ext=3 -Dpager=/usr/bin/pager -Uafs -Ud_csh -Uusesfio -Duseshrplib -Dlibperl=libperl.so.5.6.0 -Dd_dosuid -des'
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef use5005threads=undef useithreads=undef usemultiplicity=undef
    useperlio=undef d_sfio=undef uselargefiles=define 
    use64bitint=undef use64bitall=undef uselongdouble=undef usesocks=undef
  Compiler:
    cc='cc', optimize='-O2', gccversion=2.95.3 20010219 (prerelease)
    cppflags='-DDEBIAN -fno-strict-aliasing -I/usr/local/include'
    ccflags ='-DDEBIAN -fno-strict-aliasing -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    stdchar='char', d_stdstdio=define, usevfork=false
    intsize=4, longsize=4, ptrsize=4, doublesize=8
    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, usemymalloc=n, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib
    libs=-lnsl -ldl -lm -lc -lcrypt
    libc=/lib/libc-2.2.2.so, so=so, useshrplib=true, libperl=libperl.so.5.6.0
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynamic'
    cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib'

Locally applied patches:
    


@INC for perl v5.6.0:
    /usr/local/lib/perl/5.6.0
    /usr/local/share/perl/5.6.0
    /usr/lib/perl5
    /usr/share/perl5
    /usr/lib/perl/5.6.0
    /usr/share/perl/5.6.0
    /usr/lib/perl5/5.6/i386-linux
    /usr/lib/perl5/5.6
    /usr/lib/perl5/5.005/i386-linux
    .


Environment for perl v5.6.0:
    HOME=/home/simon
    LANG (unset)
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/simon/bin:/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games:/usr/local/sbin:/usr/sbin:/sbin
    PERL_BADLANG (unset)
    SHELL=/usr/local/bin/zsh

[p5pRT]

From @pjscott

At 02​:36 PM 3/4/2001 +0000, simon@​netthink.co.uk wrote​:

Maybe this is something obvious I missed, and I know I'm dealing with
somewhat undefined semantics, but...​:

$_ = "this/is/a/test"; y#/#\n#; s// /g; print

prints

t h i s
i s
a
t e s t

as I'd expect, whereas

$_ = "this/is/a/test"; s#/#\n#g; s// /g; print

prints

this
is
a
test

which I wouldn't.

No bug. The empty regular expression is defined as matching the last successfully matched regex; see perlop/"Regexp Quote-Like Operators".

The man page could stand some mention of the behavior you did expect, though :-)

Inline Patch

--- pod/perlop.pod.orig Sun Mar  4 07:40:18 2001
+++ pod/perlop.pod      Sun Mar  4 07:44:22 2001
@@ -808,6 +808,8 @@
 
 If the PATTERN evaluates to the empty string, the last
 I<successfully> matched regular expression is used instead.
+If there is no previous regular expression, successfully matched or
+otherwise, an empty PATTERN matches an empty substring.
 
 If the C</g> option is not used, C<m//> in list context returns a
 list consisting of the subexpressions matched by the parentheses in the