Regular Expressions 101

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/
^[^()\n]*+(\((?>[^()\n]|(?1))*+\)[^()\n]*+)++$
/
gm
^ asserts position at start of a line
Match a single character not present in the list below
[^()\n]
*+ matches the previous token between zero and unlimited times, as many times as possible, without giving back (possessive)
()
matches a single character in the list () (case sensitive)
\n matches a line-feed (newline) character (ASCII 10)
1st Capturing Group
(\((?>[^()\n]|(?1))*+\)[^()\n]*+)++
++ matches the previous token between one and unlimited times, as many times as possible, without giving back (possessive)
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
\( matches the character ( with index 4010 (2816 or 508) literally (case sensitive)
Atomic Group
(?>[^()\n]|(?1))*+
Non-capturing group that discards backtracking positions once matched.
*+ matches the previous token between zero and unlimited times, as many times as possible, without giving back (possessive)
1st Alternative
[^()\n]
Match a single character not present in the list below
[^()\n]
()
matches a single character in the list () (case sensitive)
\n matches a line-feed (newline) character (ASCII 10)
2nd Alternative
(?1)
(?1) recurses the 1st subpattern
\) matches the character ) with index 4110 (2916 or 518) literally (case sensitive)
Match a single character not present in the list below
[^()\n]
*+ matches the previous token between zero and unlimited times, as many times as possible, without giving back (possessive)
()
matches a single character in the list () (case sensitive)
\n matches a line-feed (newline) character (ASCII 10)
$ asserts position at the end of a line
Global pattern flags
g modifier: global. All matches (don't return after first match)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String