Lazy regex doesn't work as expected C#

Question

I have the following regex: a?\W*?b and I have a string ,.! ,b
When searching for a match I get ,.! ,b, but not just b as I expect. Why is that? How to modify the regex to get what I need?
Thank you for your help.

Answer 1

A lazy quantifier doesn't help here for what you want. Let's see what's happening.

The regex engine starts at the beginning of the string. First tries to match a. It can't, but it's no problem since the a is optional.

Then, there is a lazy \W*? so the regex engine skips it but remembers the current position.

It then tries to match b. It can't, so it backtracks and successfully matches the , with \W*?. It then goes on to try and match b (because of the lazy quantifier). It still can't and backtracks again. This repeats a few times until finally the regex engine has arrived at the b. Now the match is complete - the regex engine declares success.

So the regex works as specified - just not as intended. Now the question is: What exactly do you want the regex to do?

For example, if what you really want is:

Match b alone, unless it's preceded by a and some non-word characters, in which case match everything from a to b, then use

b|a\W*b

Answer 2

A lazy expression is only lazy from the right, i.e. it will be as short as possible by removing characters on the right, but it will not remove characters on the left.

To make the match start later, you need a greedy expression before it that swallows the characters that you don't want to match.

Alternatively, as Tim showed, you can make the match start later by only matching the first character and the following separators if the first character exists.