Regular Expressions 101

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/
@if(?:\s+|)\(([\s\S]*?)\)([\s\S]*?)+@endif
/
i
@if
matches the characters @if literally (case insensitive)
Non-capturing group
(?:\s+|)
1st Alternative
\s+
\s
matches any whitespace character (equivalent to [\r\n\t\f\v \u00a0\u1680\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
2nd Alternative null, matches any position
\( matches the character ( with index 4010 (2816 or 508) literally (case insensitive)
1st Capturing Group
([\s\S]*?)
Match a single character present in the list below
[\s\S]
*? matches the previous token between zero and unlimited times, as few times as possible, expanding as needed (lazy)
\s matches any whitespace character (equivalent to [\r\n\t\f\v \u00a0\u1680\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff])
\S matches any non-whitespace character (equivalent to [^\r\n\t\f\v \u00a0\u1680\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff])
\) matches the character ) with index 4110 (2916 or 518) literally (case insensitive)
2nd Capturing Group
([\s\S]*?)+
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
Match a single character present in the list below
[\s\S]
*? matches the previous token between zero and unlimited times, as few times as possible, expanding as needed (lazy)
\s matches any whitespace character (equivalent to [\r\n\t\f\v \u00a0\u1680\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff])
\S matches any non-whitespace character (equivalent to [^\r\n\t\f\v \u00a0\u1680\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff])
@endif
matches the characters @endif literally (case insensitive)
Global pattern flags
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
Your regular expression does not match the subject string.

Regular Expression
No Match

/
/
i

Test String

Code Generator

Generated Code

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Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report.
For a full regex reference for Swift 5.2, please visit: https://developer.apple.com/documentation/foundation/nsregularexpression