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I have a regex expression that I tested on http://gskinner.com/RegExr/ and it worked, but when I used it in my C# application it failed.
My regex expression: (?<!\d)\d{6}\K\d+(?=\d{4}(?!\d))
Text: 4000751111115425
Result: 111111
What is wrong with my regex expression?
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It's a negative lookahead, which means that for the expression to match, the part within (?!...) must not match. In this case the regex matches http:// only when it is not followed by the current host name (roughly, see Thilo's comment).
There's two ways to say "don't match": character ranges, and zero-width negative lookahead/lookbehind. Also, a correction for you: * , ? and + do not actually match anything. They are repetition operators, and always follow a matching operator.
This issue you are having is that .NET regular expressions do not support \K, "discard what has been matched so far".
I believe your regex translates as "match any string of more than ten \d digits, to as many digits as possible, and discard the first 6 and the last 4".
I believe that the .NET-compliant regex
(?<=\d{6})\d+(?=\d{4})
achieves the same thing. Note that the negative lookahead/behind for no-more-\ds is not necessary as the \d+ is greedy - the engine already will try to match as many digits as possible.
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