How do I match a expression start with $a and not contain $b after $a

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: How do I match a expression start with $a and not contain $b after $a by strat (Canon) on Jan 28, 2002 at 20:58 UTC
This isn't an answer to your question, it's just a warning about better not using $a and $b except with sort because it can cause some funny problems: `#!/opt/gnu/bin/perl -w use strict; $a = 40; # error because of use strict? no $b = $a; # error again ? no` [download] This code will not fail although neiter $a nor $b is declared despite of use strict. Best regards, `perl -e "$_=F=>y~\martinF~stronat~=>s~[^\w]~~g=>chop,print"`	[reply] [d/l] [select]
Re: How do I match a expression start with $a and not contain $b after $a by Juerd (Abbot) on Jan 28, 2002 at 18:38 UTC
`/$a(?!.*$b)/` I will have to kill those who comment on my usage of the dot star :)	[reply] [d/l]
Re (tilly) 1: Answer: How do I match a expression start with $a and not contain $b after $a by tilly (Archbishop) on Jan 28, 2002 at 18:41 UTC
At the risk of death, you need a /s modifier if you want the intermediate junk to be able to include returns. Also you should be using \Q and \E escapes. `/\Q$a\E(?!.*\Q$b\E)/s` [download] (The mistake of using $a and $b for normal use was the fault of the questioner, and not you.)	[reply] [d/l]
Re: Re: How do I match a expression start with $a and not contain $b after $a by Anonymous Monk on Jan 29, 2002 at 06:29 UTC
That regex will match "foobarfoo" if $a = "foo" and $b = "bar", but it shouldn't. See this node. -Anomo	[reply]
Re: How do I match a expression start with $a and not contain $b after $a by Anonymous Monk on Jan 29, 2002 at 06:19 UTC
It's easier to split up the task and use two regexes: `/$a/ && !/$a.?$b/s;` If you still very much want to have it in one regex you can use `/^(?>.?$a)(?!.*?$b)/s;` but that not compatible with perl 5.004 or earlier. (You can rewrite it to be compatible with perl 5.004, but I'll leave that as an exercise for you. Hint: look in the perlre manpage.) -Anomo	[reply] [d/l] [select]
Re: Re: How do I match a expression start with $a and not contain $b after $a by Anonymous Monk on Jan 29, 2002 at 06:33 UTC
I just noticed that it's not totally clear whether 'xfoobar' should match or not with $a = 'foo' and $b = 'bar'. -Anomo	[reply]