Regular Expressions 101

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/
(--.*$|\/\*[\s\S]*?\*\/)
/
gm
1st Capturing Group
(--.*$|\/\*[\s\S]*?\*\/)
1st Alternative
--.*$
--
matches the characters -- literally (case sensitive)
.
matches any character (except for line terminators)
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
$ asserts position at the end of a line
2nd Alternative
\/\*[\s\S]*?\*\/
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
\* matches the character * with index 4210 (2A16 or 528) literally (case sensitive)
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 4210 (2A16 or 528) literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
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.

Regular Expression
No Match

/
/
gm

Test String