Regexp question

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

Replies are listed 'Best First'.
Re: Regexp question by diotalevi (Canon) on Mar 30, 2003 at 14:44 UTC
"\Q" and "\E" are your friends `$tmp =~ /\Q$path\E/` Added BTW, I wonder if your $path variable is even defined all the time. Perhaps you should check for that first: `$result = defined $path and $tmp =~ /\Q$path\E/;` Added xmath had a great insight that you should be using `index()` here instead. So that becomes (assuming `$tmp` is always defined as well) `$result = defined $path and -1 != index $tmp, $path;`.	[reply] [d/l] [select]
Re: Re: Regexp question by nothingmuch (Priest) on Mar 30, 2003 at 15:28 UTC
I'd like to add: \Q and \E are a way of telling perl, within quoted text, to protect against meta characters. This is done internally by the quotemeta function. Meta characters are characters with meaning, normally beyond the seven bit ascii range. What \Q and \E, or quotemeta do is simply add a backslash before every character which can have a special meaning. An example of where this is needed is the at (@) character - it can mean an array, or just '@'. Regexes are very fragile in this sense, because many characters have a meaning. -nuffin zz zZ Z Z #!perl	[reply]
Re: Regexp question by xmath (Hermit) on Mar 30, 2003 at 16:41 UTC
If you don't want any characters inside $path to be interpreted as regex metachars.. then why don't you use the index function instead of a regex?	[reply]
Re: Regexp question by Cody Pendant (Prior) on Mar 30, 2003 at 22:16 UTC
I think this is a very useful example of something I don't have a name for, other than a "how should I use my hammer on this screw?" question. It's a perfectly valid question, but it contains what I think is an erroneous assumption -- that it's special characters inside $path which are causing the problem (and also that "%" is a regex special character, but that's a minor issue). So people are trying to help you with the \Q and \E, but that may just be confusing the issue. Special characters in $path can't "damage" your regex at all, Hotshot, they can only enhance it. They might enhance it in ways that give unexpected results, but they shouldn't be causing that error message. Your problem is that you get the "Use of uninitialized value in regexp compilation", period. Like everyone else, I think you should check what's in $path before you do the regex, but I just wanted to get a bit meta on this question, plus call attention to your misapprehension about the percent sign. -- “Every bit of code is either naturally related to the problem at hand, or else it's an accidental side effect of the fact that you happened to solve the problem using a digital computer.” M-J D	[reply]
Re: Regexp question by Abigail-II (Bishop) on Mar 31, 2003 at 09:23 UTC
The problem is that `$path` doesn't contain anything. It's undefined. Abigail	[reply] [d/l]
Re: Regexp question by aquarium (Curate) on Mar 31, 2003 at 05:20 UTC
comment out your regexp line and insert a line above it: print $path; that way your program should run, and print the value of $path which looks like being undef in your case. Chris	[reply]