Split operator in perl

binesh_28 has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Split operator in perl by BrowserUk (Patriarch) on Dec 06, 2010 at 06:22 UTC
Because your regex contains capturing parens, split will return both the separated bits and the captured separators: `print split ',', 'a,b,c';; a b c print split '(,)', 'a,b,c';; a , b , c` [download] However, as you have a quantifier applied to the capturing parens, the regex will match multiple consecutive characters matching `(a\|b)+` but will only retain the last one captured. Hence although the regex matches & captures each of the four characters in 'abab', only the final 'b' will be retained and returned. `print 'abcd' =~ m[(.)+];; d` [download] Putting that all together, and the delimiters used to split the string are as indicated by '[]': `"[a]12cd[abab]"`; but only the last single character of each delimiter is retained, hence:'(a)', '12cd', '(b)' 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.	[reply] [d/l] [select]
Re: Split operator in perl by ikegami (Patriarch) on Dec 06, 2010 at 06:11 UTC
Using a quantifier (+,*,?,{m,n}) on a capture rarely makes sense. If you didn't want to capture: /`(?:a\|b)+`/ or /`[ab]+`/. If you wanted to capture the whole separator: /`((?:a\|b)+)`/ or /`([ab]+)`/. Then there's the fact that your separator is probably not really a separator since one a matching pattern is found at the start of the string. This will cause you to have a leading empty field. What are you expecting for output?	[reply] [d/l] [select]
Re^2: Split operator in perl by binesh_28 (Initiate) on Dec 06, 2010 at 07:58 UTC
I was pretty confused trying to understand the results from that operation. I wasn't trying to accomplish any other task. That was one of the examples quoted as part of perl docs, and as a beginner to perl, I was having a hard time trying to figure out the results. Thanks for your help.	[reply]
Re^3: Split operator in perl by ikegami (Patriarch) on Dec 06, 2010 at 16:23 UTC
As a beginner to Perl, maybe you shouldn't worry about what weird results split will give if you give it weird inputs. I bet most Perl experts would have to guess at what 'ab'=~/(a\|b)+/ returns (although they would likely guess correctly). More important would be to learn how to use split properly. The regular expressions should identify the separator. In your case, it doesn't appear to be used to match a separator, so split is probably not the best tool for the job. You'll rarely want to use captures in a split pattern. When you do, what the captures match is returned in split's result. Using a quantifier on a capture doesn't make sense. If you want to group things in patterns, use `(?:...)` instead of `(...)`. The latter is much slower since it remembers what the parens matched, and has side effects in split.	[reply] [d/l] [select]
Re: Split operator in perl by perl_lover (Chaplain) on Dec 06, 2010 at 06:20 UTC
If you want to understand how the regex execution is happening, just add this line to your code `use re 'debug';` [download] use Perl; Perl4Everything	[reply] [d/l]
Re: Split operator in perl by suhailck (Friar) on Dec 06, 2010 at 06:21 UTC
See the below examples and find the differences Case 1: `perl -e 'print map {++$count," : ",$_,"\n"}split /(a\|b)+/,"a12cdabab"' 1 : 2 : a 3 : 12cd 4 : b` [download] Case 2: `perl -e 'print map {++$count," : ",$_,"\n"}split /((?:a\|b)+)/,"a12cdab +ab"' 1 : 2 : a 3 : 12cd 4 : abab` [download] Case 3: `perl -e 'print map {++$count," : ",$_,"\n"}split /(?:a\|b)+/,"a12cdabab +"' 1 : 2 : 12cd` [download]	[reply] [d/l] [select]