Regular Expressions 101

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/
^(?:https?:\/\/(www|m|)\.(youtube\.com\/watch\?v=|youtu.be\/))([a-zA-Z\d_-]{11})$
/
gm
^ asserts position at start of a line
Non-capturing group
(?:https?:\/\/(www|m|)\.(youtube\.com\/watch\?v=|youtu.be\/))
http
matches the characters http literally (case sensitive)
s
matches the character s with index 11510 (7316 or 1638) literally (case sensitive)
: matches the character : with index 5810 (3A16 or 728) 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|m|)
1st Alternative
www
www
matches the characters www literally (case sensitive)
2nd Alternative
m
m matches the character m with index 10910 (6D16 or 1558) literally (case sensitive)
3rd Alternative null, matches any position
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
2nd Capturing Group
(youtube\.com\/watch\?v=|youtu.be\/)
1st Alternative
youtube\.com\/watch\?v=
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)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
watch
matches the characters watch literally (case sensitive)
\? matches the character ? with index 6310 (3F16 or 778) literally (case sensitive)
v=
matches the characters v= literally (case sensitive)
2nd Alternative
youtu.be\/
youtu
matches the characters youtu literally (case sensitive)
. matches any character (except for line terminators)
be
matches the characters be literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
3rd Capturing Group
([a-zA-Z\d_-]{11})
Match a single character present in the list below
[a-zA-Z\d_-]
{11} matches the previous token exactly 11 times
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
\d matches a digit (equivalent to [0-9])
_ matches the character _ with index 9510 (5F16 or 1378) literally (case sensitive)
- 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!
$ 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.

Regular Expression
No Match

/
/
gm

Test String