Regular Expressions 101

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/
^(https:|http:|www\.)\S*
/
gm
^ asserts position at start of a line
1st Capturing Group
(https:|http:|www\.)
1st Alternative
https:
https:
matches the characters https: literally (case sensitive)
2nd Alternative
http:
http:
matches the characters http: literally (case sensitive)
3rd Alternative
www\.
www
matches the characters www literally (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
\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 previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
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