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Thread: Regex: find all possible matches
 10132010, 12:43 PM #1Member
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Regex: find all possible matches
Hi,
this is a problem that has been bothering me a while now:
imagine you have a string (e.g. "___a___b___c___") and you have a regex (e.g. "([abc]).*([abc])") and you want to find all possible combinations of matching groups (e.g. ab, ac, bc).
You can play around with greedy and non greedy quantifiers (e.g. "([abc]).*([abc])" and "([abc]).*?([abc])") which will find some of this combinations (e.g. ab and ac)  but not all (in this case not bc).
What I've been doing till now is basically this:
Java Code:String haystack = "___a___b___c___"; for (int j = 0; j < 100; j++) { for (int i = 0; i < 100; i++) { String pattern = "^.{" + j + "}([abc]).{" + i + "}([abc])"; Pattern p = Pattern.compile(pattern); Matcher m = p.matcher(haystack); while (m.find()) { System.out.printf("%2d,%2d : %s  %s\n", j, i, m.group(1), m.group(2)); } } }
Java Code:3, 3 : a  b 3, 7 : a  c 7, 3 : b  c
However it does this by trying all possible expressions (in this case 100*100 = 10000 times  yes you could use less than 100, but you get the point).
Is there a more elegant (i.e. faster) solution to this?
Regards,
Alessio
 10182010, 01:19 PM #2Member
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*bump* (sorry, about that)
 10182010, 07:30 PM #3
This is probably how I'd do it... it's not much "faster" but it prevents hardcoding values (like 100).
Java Code:String haystack = "___a___b___c___"; for (int i = 0; i < haystack.length()  1; i++) { for (int j = i + 1; j < haystack.length(); j++) { String pattern = "^([abc]).*([abc])$"; Pattern p = Pattern.compile(pattern); Matcher m = p.matcher(haystack.substring(i,j)); while (m.find()) { System.out.printf("%2d,%2d : %s  %s\n", i,j, m.group(1), m.group(2)); } } }
Java Code:_ __ ___ ___a ___a_ ___a__ ___a___ ___a___b ___a___b_ ... _ _c _c_ _c__ c c_ c__ _ __ _
I'm kind of convinced there's a faster way to do this yet, so I'm still looking. This was worth posting I thought though.
 10192010, 01:10 PM #4Member
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