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This should be a pretty simple regex question but I couldn't find any answers anywhere. How would one make a regex, which matches on either ONLY 2 characters, or at least 4 characters. Here is my current method of doing it (ignore the regex itself, that's besides the point):
[A-Za-z0_9_]{2}|[A-Za-z0_9_]{4,} However, this method takes twice the time (and is approximately 0.3s slower for me on a 400 line file), so I was wondering if there was a better way to do it?
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$ means "Match the end of the string" (the position after the last character in the string). Both are called anchors and ensure that the entire string is matched instead of just a substring.
Basically (0+1)* mathes any sequence of ones and zeroes. So, in your example (0+1)*1(0+1)* should match any sequence that has 1. It would not match 000 , but it would match 010 , 1 , 111 etc. (0+1) means 0 OR 1.
The { n , m } quantifier matches the preceding element at least n times, but no more than m times, where n and m are integers. { n , m } is a greedy quantifier whose lazy equivalent is { n , m }? .
Optimize the beginning, and anchor it.
^[A-Za-z0-9_]{2}(?:|[A-Za-z0-9_]{2,})$ (Also, you did say to ignore the regex itself, but I guessed you probably wanted 0-9, not 0_9)
EDIT Hm, I was sure I read that you want to match lines. Remove the anchors (^$) if you want to match inside the line as well. If you do match full lines only, anchors will speed you up (well, the front anchor ^ will, at least).
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