Regex to capture an optional group in the middle of a block of input

Question

I\'m stuck on a RegEx problem that\'s seemingly very simple and yet I can\'t get it working.

Suppose I have input like this:

Some text %interestingbi         


        

Answer 1

Try this:

%interestingbit%(?:(.+)(?OPTIONAL_THING))?(.+?)%anotherinterestingbit%

First there's a non-capturing group which matches .+OPTIONAL_THING or nothing. If a match is found, there's the named group inside, which captures OPTIONAL_THING for you. The rest is captured with .+?%anotherinterestingbit%.

[edit]: I added a couple of parentheses for additional capture groups, so now the captured groups match the following:

• $1 : text before OPTIONAL_THING or nothing
• $2 or $optionalCapture : OPTIONAL_THING or nothing
• $3 : text after OPTIONAL_THING, or if OPTIONAL_THING is not found, the full text between %interestingbit% and %anotherinterestingbit%

Are these the three matches you're looking for?