Quoting metacharacters in regexen

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

need help with something probably fairly simple. i am using m to search for a variable $string, like this:

$LINES[$i] =~ /$string/

however $string contains characters like + and ^ that are evaluated by the m// function after $string is interpolated, and I do not want this to happen.

On the perlman page on this website it says to use the /Q to prevent this second level of interpolation, but I tried using:

/\Q$string\E/
   and
/$\Qstring\E/
[download]

and also I tried escaping the non-word characters (another suggestion I found on another website) before I performed the match, like this:

$string =~ s/(W)/\\$1/g

but none of these worked b/c every time I tried it it gave me the same error:

Quantifier follows nothing before HERE mark in regex m/+ << HERE 5^7/

where $string in this instance was +5^7
does anybody know how to fix this?

20031124 Edit by BazB: Changed title from 'newbie question'

Comment on Quoting metacharacters in regexen Select or Download Code

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Re: Quoting metacharacters in regexen by Roger (Parson) on Nov 24, 2003 at 00:38 UTC
I have written the following demo program but was delayed for almost an hour because I had to attend the Monday morning's manager's meeting. Anyway, I will post it here anyway, since no monks have shown you a search and replace example yet. :-) `use strict; my $string = '+5^7'; my $line = '((1+2)-4)(2+5^7)'; # insert \ in front of pattern $line =~ s/(\Q$string\E)/\\$1/; print "$line\n";` [download] Basically you use \Q and \E to quote meta characters, or use the 'quotemeta' function. And the output is - `((1+2)-4)(2\+5^7)` [download]	[reply] [d/l] [select]
Re: Quoting metacharacters in regexen by Anonymous Monk on Nov 24, 2003 at 00:09 UTC
Either using quotemeta or "\Q \E" solves your problem. `$_ = "+5^7"; $pattern = "+5^7"; print "ok\n" if m/\Q$pattern\E/;' $_ = "+5^7"; $pattern = quotemeta "+5^7"; print "ok\n" if m/$pattern/;'` [download]	[reply] [d/l]
Re: Quoting metacharacters in regexen by evilsizord (Initiate) on Nov 24, 2003 at 00:58 UTC
ahah thanks guys. i found out that the \Q didn't work the first time because i also used it in an s// function, and when i only did the left side, i.e. : `s/\Q$string1\E/$string2/` it works! I guess s// doesn't try to evaluate the replacement string as a regular expression after it interpolates it.	[reply] [d/l]
Re: Quoting metacharacters in regexen by jZed (Prior) on Nov 24, 2003 at 00:06 UTC
perldoc -f quotemeta	[reply]