Regular Expressions 101

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`
(?:\s|^|\(|\[)(#[0-9]+)(?:\s|$|\)|\]|\.|\,(?:\s|$))
`
smg
Non-capturing group
(?:\s|^|\(|\[)
1st Alternative
\s
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
2nd Alternative
^
^ asserts position at start of a line
3rd Alternative
\(
\( matches the character ( with index 4010 (2816 or 508) literally (case sensitive)
4th Alternative
\[
\[ matches the character [ with index 9110 (5B16 or 1338) literally (case sensitive)
1st Capturing Group
(#[0-9]+)
# matches the character # with index 3510 (2316 or 438) literally (case sensitive)
Match a single character present in the list below
[0-9]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
Non-capturing group
(?:\s|$|\)|\]|\.|\,(?:\s|$))
1st Alternative
\s
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
2nd Alternative
$
$ asserts position at the end of a line
3rd Alternative
\)
\) matches the character ) with index 4110 (2916 or 518) literally (case sensitive)
4th Alternative
\]
\] matches the character ] with index 9310 (5D16 or 1358) literally (case sensitive)
5th Alternative
\.
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
6th Alternative
\,(?:\s|$)
\, matches the character , with index 4410 (2C16 or 548) literally (case sensitive)
Non-capturing group
(?:\s|$)
1st Alternative
\s
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
2nd Alternative
$
$ asserts position at the end of a line
Global pattern flags
s modifier: single line. Dot matches newline characters
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.

Regular Expression
No Match

`
`
smg

Test String