.net regex with condition lookbehind and capture group

Question

Pattern: a(?(?<! ) )b (c)

Input: a b c

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

It matches correct, but the capture group $1 is empty (instad of containing c).

Is this a problem with .net regex or am I missing something?

Example: http://regexstorm.net/tester?p=a(%3f(%3f%3C!+)+)b+(c)&i=a+b+c

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(?(?<! ) )b (c)()

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(?((?<! )) )b (c)
   

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

   a(?(?<! ) )b (?<first>c)
   

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

   a(?(?<! ) )(b) (c)
   

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

   a(?( ) )b (c)