in reply to A regexp to parse nested brackets containing strings

davido has good advice with Text::Balanced. If you don't use that, at least get out of a totally regex solution and break things apart with Perl code managing the regexs, that way you can see what's going on as you pick things apart.

If you're like me, it's also tough to just leave behind the ideas of "it should have worked", and "why didn't it". Here's my shot at helping with that:

The $innerRe doesn't have a "^" in it to anchor it at the beginning of the string. That means it can skip over any characters it comes accross, including open parens and quotes, until it finds something it wants, like the open parens inside the quote. Then all it has to do is find a matching closing paren and it's done. That's why the part of $innerRe that handles embedded quotes isn't doing it's job, it never comes into play.

That leaves a follow-on question. Shouldn't we just achor the beginning of $innerRe with a "^" followed by "[^()]*" to account for non-paren text? I tried that, but $innerRe is also used recursively later on in the match, and putting "^" at the beginning of $innerRe means you effectively have two occurences of "^", one at the beginning and another later in the regex. So you can't make $InnerRe anchorred at the beginning, and not anchorred at the beginning, all in the same match invocation. You need to do a match, pull the resulting string out, then match that, so that "^" then refers to the string bound to the second match.

And that's what Text::Balanced is for. It will also help handle the case where you've got a double quoted string, where that string contains an escaped double quote. That can be rough to handle in a regex that's trying to do everything else.

Update: Added escapes for square brackets (thanks to Fletch). Added "in it" to second paragraph to be clearer (after seeing ikegami's reading of it).

  • Comment on Re: A regexp to parse nested brackets containing strings

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Re^2: A regexp to parse nested brackets containing strings
by ikegami (Patriarch) on Sep 23, 2006 at 21:35 UTC

    So you can't make $InnerRe anchorred at the beginning, and not anchorred at the beginning, all in the same match invocation.

    It's not a problem. My solution even does this. 

    $\ = "\n";

my $re;
$re = qr/ a (??{ $re }) c | b /x;

print 'aabcc'  =~ /^$re/ ||0;  # 1
print '!aabcc' =~ /^$re/ ||0;  # 0  First call is anchored to start.
print 'a!abcc' =~ /^$re/ ||0;  # 0  Recursive call is anchored to pos.
print 'aa!bcc' =~ /^$re/ ||0;  # 0  Recursive call is anchored to pos.

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      Looks like I wasn't specific enough in my writeup. Yes, you can put a "^" anchor in the top-level invocation of $re, but not in the $re itself. For the OP's original problem, where he is looking for the innermost parenthesese that are not inside a quote, you want the "^L in the $re. If you don't put it there, the invocation of $re from within the first invocation of $re can just skip over text to find the parentheses within the quotes. However, "^" doesn't do what you need there, because it doesn't mean the beginning of where $re was invoked in each recursive iteration, it means the beginning of this invocation of the regex parser. At least that's what it looks like from the behavior.
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        If you don't put it there, the invocation of $re from within the first invocation of $re can just skip over text to find the parentheses within the quotes.

        No, it can't skip. That's what I was showing. If it could skip, all the regexp in my parent post would match, since they would skip over the !. When using (??{ $re }), matching $re is anchored at the current pos..

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