Regular Expressions 101

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/
^(?:https:\/\/)?((www\.)?youtube\.com|youtu\.?be)\/.+$
/
gm
^ asserts position at start of a line
Non-capturing group
(?:https:\/\/)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
https:
matches the characters https: literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
1st Capturing Group
((www\.)?youtube\.com|youtu\.?be)
1st Alternative
(www\.)?youtube\.com
2nd Capturing Group
(www\.)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
www
matches the characters www literally (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
youtube
matches the characters youtube literally (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
com
matches the characters com literally (case sensitive)
2nd Alternative
youtu\.?be
youtu
matches the characters youtu literally (case sensitive)
\.
matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
be
matches the characters be literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
.
matches any character (except for line terminators)
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
$ 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