C# Regex Match parenthesis group except if it contains specified word

user_17118493 0
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I have a long string:

(Today is a blue day) (Today is a good day) (Today is a BAD day) (Today is a green day) (Today is a blue day)

I want to match the parentheses groups, except if it contains the capitalized word. The word will always be fully capitalized, but may not be the only fully capitalized word - but will be the only word that is exactly BAD.

I have a very long string and I want to change the parentheses groups that do not contain the word BAD while leaving BAD alone. I was hoping to avoid iterating over every single parentheses group to check if it contains BAD.

This: \(.+?\)

Will match my parentheses groups.

I have tried:

\(.+?(?=\bBAD\b).+?\) - this matches every group up to the group containing BAD.

(?=\bBAD\b).+?\) - this matches the end of the group "BAD day)"

I tried a few variations of negative lookbehinds but could not get them to provide a result.

I know this works:

\(.[^BAD]+?\)

Until you include (Today is a Blue day) - and then it fails.

Anyone know an effective way to do this?

regex c#
2 Answers

You can use

\((?>([^()]*\bBAD\b)?)[^()]*\)(?(1)(?!))

See the .NET regex demo. Details:

  • \( - a ( char
  • (?>([^()]*\bBAD\b)?) - an atomic group (that disallows re-trying its pattern when backtracking occurs): zero or more chars other than ) and ( and then a whole word BAD, all captured into Group 1
  • [^()]* - zero or more chars other than ( and )
  • \) - a ) char
  • (?(1)(?!)) - if Group 1 was matched, trigger backtracking (here, it will fail the match since we used an atomic group before).

See the C# demo:

var text = "(Today is a blue day) (Today is a good day) (Today is a BAD day) (Today is a green day) (Today is a blue day)";
var matches = Regex.Matches(text, @"\((?>([^()]*\bBAD\b)?)[^()]*\)(?(1)(?!))")
    .Cast<Match>()
    .Select(x => x.Value)
    .ToList();

Output:

(Today is a blue day)
(Today is a good day)
(Today is a green day)
(Today is a blue day)
user_38329700

This part (?=\bBAD\b).+?\) asserts BAD to the right and then matches as least as possible till the next ). It can also be written without the lookahead \bBAD\b.+?\)

This part [^BAD] matches any character except the characters B A D

You can use the opposite using a negative lookahead instead to asser that BAD is not between parenthesis, and you might also add word boundaries \b to prevent a partial match.

\((?![^()]*\bBAD\b[^()]*\))[^()]*\)

The pattern matches:

  • \( Match (
  • (?![^()]*\bBAD\b[^()]*\)) Negative lookahead, assert not optional parenthesis followed by the word BAD till the first closing parenthesis to the right
  • [^()]* Match 0+ times any char except ( ) using a negated character class
  • \) Match )

Regex demo

user_54249880
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