Regex to find matches from a string having multiple square brackets

Question

I have a paragraph like below:

Some wording for testing [!#today] where the [!condition] does not satisfy with this verbiage [!ShowElemIf://Student/FullName; [[[Text not recognized fully]]] ;/First Name] But simple tags found having age [!ShowElemIf://Student/Age;xml//Student/DOB/@formatted;y]

I need to find all the placeholders/tags from above which are like: [!tag] using C#. I tried for a regex but it is not able to find the tag having "FullName" word as highlighted in bold above.

 List<string> tags = Regex.Matches( sampleText.Replace(Environment.NewLine, ""), @"\\[!([^]]+)\\] ") .Cast<Match>() .Select(x => x.Groups[1].Value) .ToList(); 

Using this RegEx I can find below but not the highlighted one.

1. today
2. condition
3. ShowElemIf://Student/Age;xml//Student/DOB/@formatted;y

Answer 1

You need to use balancing groups available in .NET regex:

@"\[!((?:[^][]+|(?<o>\[)|(?<-o>)])*(?(o)(?!)))]"

See the regex demo

Details :

• \\[! - a [! substring
• ((?:[^][]+|(?<o>\\[)|(?<-o>)])*(?(o)(?!))) - Capturing group 1 matching
  • (?: - a non-capturing group with 3 alternatives:
    • [^][]+| - 1 or more chars other than [ and ] , or
    • (?<o>\\[)| - a [ pushed into "o" group stack, or
    • (?<-o>)] - a ] is subtracted from the "o" group stack
  • )* - zero or more occurrences
  • (?(o)(?!))) - a conditional construct checking if the "o" group stack is empty. If it is empty, matching goes on, else, no match is returned.
• ] - a literal closing ] symbol.