Regular Expressions 101

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/
^( ?[-*_]){3,} ?[ \t]*$
/
gm
^ asserts position at start of a line
1st Capturing Group
( ?[-*_]){3,}
{3,} matches the previous token between 3 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
matches the character with index 3210 (2016 or 408) literally (case sensitive)
Match a single character present in the list below
[-*_]
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
This hyphen is treated literally, which might be confusing for others. Consider escaping it or placing at the start or end of the class!
*_
matches a single character in the list *_ (case sensitive)
matches the character with index 3210 (2016 or 408) literally (case sensitive)
Match a single character present in the list below
[ \t]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
matches the character with index 3210 (2016 or 408) literally (case sensitive)
\t matches a tab character (ASCII 9)
$ 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