Question

Consider such code:

Match match = new Regex("^bar").Match(some_string,3);

I would like to match some_string but not from the beginning of it, but starting from given position. The catch is, I would like to make a match exactly from that position, thus the anchor.

Unfortunately, it does not work. MS regex does not work with chunks of the string as strings on their own, so ^ matches only the one and true beginning of the string not of starting position.

The easy workaround is to write this in such way:

Match match = new Regex("^bar").Match(some_string.Substring(3));

The downsize — speed (i.e. lack of it).

So my question is this — how to match anchored regex in the middle of the string, fast?

Answer 1

.NET has a separate anchor for the position where the engine starts (usually the end of the last match, in your case the offset): \G.

So you should be able to use the pattern @"\Gbar".

Source (MSDN)

If you want to restrict a match so that it begins at a particular character position in the string and the regular expression engine does not scan the remainder of the string for a match, anchor the regular expression with a \G (at the left for a left-to-right pattern, or at the right for a right-to-left pattern). This restricts the match so it must start exactly at startat.