.net regex with condition lookbehind and capture group

Question

Pattern: a(?(?

Input: a b c

Desription: Condition should match space, if lookbehind is not a space.

It ma

Answer 1

I'm not sure if this behavior is documented or not (if yes then I didn't find it) but using a conditional construct including an explicit zero-width assertion as its expression (?(?=expression)yes|no) overrides the very next numbered capturing group (empties it). You can confirm this by running below RegEx:

a(?(?

Four ways to overcome this issue:

1. Enclosing expression in parentheses noted by @DmitryEgorov (that also keeps second capturing group intact) and is not included in result - the right way:

   a(?((?

2. As this behavior is only applied to unnamed capturing groups (default) you can get expected result using a named capturing group:

   a(?(?c)
   

3. Adding an extra capturing group where ever you like between (c) and conditional:

   a(?(?

4. Avoiding such an expression if possible. E.g:

   a(?( ) )b (c)