Regex curly bracket ambiguity

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

Hi. I'm having an ambiguity problem with the {} brackets in perl regex. It thinks I'm trying to reference a nonexistent hash when really I just have variables for the desired match and the number of times I want to match.

my code:

$string = hithereHelloHellohithere
$match = Hello;
$repeat = 2;

$string =~ m/$match{$repeat}/;
[download]

Gives me errors indicating it's looking for a hash that doesn't exist.

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Re: Regex curly bracket ambiguity by ysth (Canon) on Jun 21, 2007 at 16:03 UTC
To indicate where a variable ends in a regex, you surround the variable with {}: `$string =~ m/${match}{$repeat}/;`. That keeps it from thinking you mean %match, but gives you `m/Hello{2}/` (match "Hell", followed by two "o"s). To make the repeat apply to the whole $match, you need to group it: `m/(?:${match}){$repeat}/`, which makes the added {} unnecessary, leaving you with `m/(?:$match){$repeat}/`.	[reply] [d/l] [select]
Re^2: Regex curly bracket ambiguity by Anonymous Monk on Jun 21, 2007 at 18:08 UTC
Thank you that was incredibly helpful. so now that I have regular brackets around (?:$match); what do I do if I want to capture the whole thing? eg with: m/((?:$match){$repeat})/ will $1 give me everything or just $match back?	[reply]
Re^3: Regex curly bracket ambiguity by Roy Johnson (Monsignor) on Jun 21, 2007 at 18:11 UTC
`$1` will give you the whole match. `(?:)` is a non-capturing grouping, so it does not affect `$1`, etc. If you used regular parens around `$match`, it would be returned in `$2`. Caution: Contents may have been coded under pressure.	[reply] [d/l] [select]
Re^4: Regex curly bracket ambiguity by ysth (Canon) on Jun 21, 2007 at 18:23 UTC
Re: Regex curly bracket ambiguity by halley (Prior) on Jun 21, 2007 at 15:05 UTC
It's pretty easy to find some other syntax that will not change the meaning of your pattern, but give Perl more clues as to your intent. For example, a `(?:...)` non-capturing group. `$string =~ m/(?:$match){$repeat}/;` [download] -- `[ e d @ h a l l e y . c c ]`	[reply] [d/l] [select]
Re: Regex curly bracket ambiguity by naikonta (Curate) on Jun 21, 2007 at 18:36 UTC
This was just discussed recently from another point of view. Open source softwares? Share and enjoy. Make profit from them if you can. Yet, share and enjoy!	[reply]