flip flop behaviour of regex match

harsha.reddy has asked for the wisdom of the Perl Monks concerning the following question:

Hi All

#!/usr/bin/perl -w

sub func
{
    my $CmdSep = ($^O =~ m/MSWin32/ig)? "\&" : "\;";
    
    print "The Command Seperator is: " . $CmdSep ."\n";
}

func();
func();
[download]

The output of this snippet is strange:

perl test.pl
The Command Seperator is: &
The Command Seperator is: ;

This is a very strange behaviour, I am confused that this is a feature or a BUG ;)

Tested using:

This is perl, v5.8.8 built for MSWin32-x86-multi-thread
(with 50 registered patches, see perl -V for more detail)

Copyright 1987-2006, Larry Wall

Binary build 820 274739 provided by ActiveState http://www.ActiveState.com
Built Jan 23 2007 15:57:46

Comment on flip flop behaviour of regex match Download Code

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Re: flip flop behaviour of regex match by ikegami (Patriarch) on Oct 10, 2007 at 14:15 UTC
You're using the `g` option on the match operator when you don't mean to do so. `m//g` acts as an iterator in scalar context. `$_ = 'abc'; print /(.)/g ? $1 : '[]', "\n"; # a print /(.)/g ? $1 : '[]', "\n"; # b print /(.)/g ? $1 : '[]', "\n"; # c print /(.)/g ? $1 : '[]', "\n"; # [] print /(.)/g ? $1 : '[]', "\n"; # a` [download] `$_ = 'abc'; print /(.)/ ? $1 : '[]', "\n"; # a print /(.)/ ? $1 : '[]', "\n"; # a print /(.)/ ? $1 : '[]', "\n"; # a print /(.)/ ? $1 : '[]', "\n"; # a print /(.)/ ? $1 : '[]', "\n"; # a` [download]	[reply] [d/l] [select]
Re: flip flop behaviour of regex match by mwah (Hermit) on Oct 10, 2007 at 14:36 UTC
As ikegami pointed out, /g in scalar context doesn't reset pos. Your example will work fine after some small modifications, eg.: `... sub func { print 'The Command Seperator is: ', $^O =~ /MSWin32/i ? '&' : ';', " +\n" } func(); func(); ...` [download] <Addendum> After thinking about an idiomatic switch between alternatives I got this one, which I like somehow: `sub func { print 'The Command Seperator is: ', qw(; &)[ $^O =~ /MSWin32/i ], "\n" }` [download] which evaluates the regex in scalar context as 0/1 (array index of the qw...) </Addendum> Regards mwa	[reply] [d/l] [select]
Re^2: flip flop behaviour of regex match by harsha.reddy (Acolyte) on Oct 10, 2007 at 14:54 UTC
Hi ikegami and mwa Wow awesome!! this is a super cool feature. Thanks a lot.	[reply]