Regular expression with (?:

Question

I am new to regular expression and trying to find out what this means.

(?:(?:^KC[\\x00-\\xff]{50}))

Upon looking up online, ?: means no backtrace, I am not sure what that means? Also from ^ , does that mean a line that does not contain "KC...." for 50 character long?

`(?:` is the start of a non-capturing group, but the regex you've shown is invalid. — Philippe Leybaert, Jul 11 '12 at 19:59
Oops, i didint finish the regx, i simply added end of ). But, what does it mean when you say non-capturing group? thanks. — Tony, Jul 11 '12 at 20:04
It means that it doesn't get added to the groups collection in the match, so you can't use it in a replacement expression — Philippe Leybaert, Jul 11 '12 at 20:05
The double `(?:` still makes no sense. It's syntactically valid, but useless. — user123444555621, Jul 11 '12 at 20:09
possible duplicate of [Non capturing group?](http://stackoverflow.com/questions/3512471/non-capturing-group) — Bergi, Jul 11 '12 at 20:20

score 2 · Accepted Answer · answered Jul 11 '12 at 20:02

When you use brackets () in regex, you can use references (\1..\9) to captured groups further in the regex. Example: (a|b)_\1 will match 'a_a' and 'b_b'.

?: means that the captured group won't have a number (actually, it's better to say that it won't be captured, it's just a group).

^ means negation in character classes (in []). Outside [] it means beginning of the line.

Regular expression with (?:

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