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

Why do you have the extra set of parentheses?

Try this:

%interestingbit%.+?(?<OptionalCapture>OPTIONAL_THING)?.+?%anotherinterestingbit%

Or maybe this will work:

%interestingbit%.+?(?<OptionalCapture>OPTIONAL_THING|).+?%anotherinterestingbit%

In this example, the group captures OPTIONAL_THING, or nothing.

Answer 2

My thoughts are along similar lines to Niko's idea. However, I would suggest placing the 2nd .+? inside the optional group instead of the first, as follows:

%interestingbit%.+?(?:(?<optionalCapture>OPTIONAL_THING).+?)?%anotherinterestingbit%

This avoids unnecessary backtracking. If the first .+? is inside the optional group and OPTIONAL_THING does not exist in the search string, the regex won't know this until it gets to the end of the string. It will then need to backtrack, perhaps quite a bit, to match %anotherinterestingbit%, which as you said will always exist.

Also, since OPTIONAL_THING, when it exists, will always be before %anotherinterestingbit%, then the text after it is effectively optional as well and fits more naturally into the optional group.

Answer 3

Try this:

%interestingbit%(?:(.+)(?<optionalCapture>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?