Regular Expressions 101

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`
(?:^|\s)(\*\*\*)(?:$)
`
gmi
Non-capturing group
(?:^|\s)
1st Alternative
^
^ asserts position at start of a line
2nd Alternative
\s
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
1st Capturing Group
(\*\*\*)
\* matches the character * with index 4210 (2A16 or 528) literally (case insensitive)
\* matches the character * with index 4210 (2A16 or 528) literally (case insensitive)
\* matches the character * with index 4210 (2A16 or 528) literally (case insensitive)
Non-capturing group
(?:$)
$ 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)
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

`
`
gmi

Test String