Regex priorities.

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

Hi Guys,

I have a regex designed to pick out some XML tags.

A typical string might look like this:

<COMMA><EXHIBITING></EXHIBITING></COMMA>
[download]

I want to process the smallest and inner-most pair of tags first (ie, in this case <EXHIBITING></EXHIBITING>).

I am using the regex:


my $r =  '\<(\D+)\>(.*?)(\<\/\1\>)';
while ($loc_diagnoses_text =~ m/$r/gi){
    ...  processing stuff ....
}
[download]

But it is processing the <COMMA><COMMA> pair first. How do I fix this?

Regards

Steve

Comment on Regex priorities. Select or Download Code

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Re: Regex priorities. by BrowserUk (Patriarch) on May 28, 2012 at 01:20 UTC
Does this match your expectations? `$s = '<COMMA>pre-stuff<EXHIBITING>some stuff</EXHIBITING>post-stuff</C +OMMA>';; print "$1 :: $2" while $s =~ s[<(\D+)>([^<]?)</\1>][]gi;; EXHIBITING :: some stuff COMMA :: pre-stuffpost-stuff` [download] Of course, it fails horribly if your non-tag content contains '<': `$s = '<COMMA>pre-stuff<EXHIBITING>some <= stuff</EXHIBITING>post-stuff +</COMMA>';; print "$1 :: $2" while $s =~ s[<(\D+)>([^<]?)</\1>][]gi;; {zilch here}` [download] With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. The start of some sanity?	[reply] [d/l] [select]
Re^2: Regex priorities. by Steve_BZ (Chaplain) on May 28, 2012 at 15:01 UTC
Hi BrowserUk, Thanks for that. It worked perfectly. I decided to change "<" for "/" because I use it less, in fact I do use "<" for other reasons, but not "/", so I ended up with: `my $r = q( <(\D+)> # Opening tag <....> ([^/]*?) # stuff in the middle which does not h +ave the closing tab character '/' (<\/\1>) # closing tag of same type as opening +tag </....>. ); while ($text =~ m/$r/gix){ ... processing ... }` [download] Thanks for your help. Regards Steve	[reply] [d/l]
Re: Regex priorities. by Anonymous Monk on May 28, 2012 at 01:28 UTC
How do I fix this? I read you fix it with "recursive regexes" but I'm not sure that maps well/easy to xml. See ** Eternal question of parsing parentheses Re^2: parsing XML fragments (xml log files) with... a regex recursive algorithm for nested data structures balanced parens regexp hangs - solved A regexp to parse nested brackets containing strings Elegant examples to parse parenthesised strings Regexp::Common balanced curlies substitution RFC: A walkthrough from JSON ABNF to Regexp::Grammars I'm the kind that would just use a parser :)	[reply]
Re^2: Regex priorities. by Steve_BZ (Chaplain) on May 28, 2012 at 15:05 UTC
Hi Anon, Thanks for this. I did in fact read most of the links you so kindly posted. I also thought it was a bit like a compiler problem and parsing was a potential solution, but I thought it would take longer. I was quite interested in how you would have parsed it. Regards Steve	[reply]
Re^3: Regex priorities. by Anonymous Monk on Jun 01, 2012 at 06:38 UTC
Well personally, if its actual XML I'd use something like Re^3: parse xml and store data in array of hashesh -- as you can see from the path trace, its a depth first search	[reply]

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