Regular Expressions 101

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/
(?:https?:\/\/)?(?:www.|web.|m.)?(?:facebook|fb).com\/(?:video.php\?v=\/?|\?v=\/?|watch\/\?v=\/?|watch\/\?v=\/?)|\S+\/videos\/?
/
gm
1st Alternative
(?:https?:\/\/)?(?:www.|web.|m.)?(?:facebook|fb).com\/(?:video.php\?v=\/?|\?v=\/?|watch\/\?v=\/?|watch\/\?v=\/?)
Non-capturing group
(?:https?:\/\/)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
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)
Non-capturing group
(?:www.|web.|m.)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
1st Alternative
www.
www
matches the characters www literally (case sensitive)
. matches any character (except for line terminators)
2nd Alternative
web.
web
matches the characters web literally (case sensitive)
. matches any character (except for line terminators)
3rd Alternative
m.
m matches the character m with index 10910 (6D16 or 1558) literally (case sensitive)
. matches any character (except for line terminators)
Non-capturing group
(?:facebook|fb)
1st Alternative
facebook
facebook
matches the characters facebook literally (case sensitive)
2nd Alternative
fb
fb
matches the characters fb literally (case sensitive)
. matches any character (except for line terminators)
com
matches the characters com literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
Non-capturing group
(?:video.php\?v=\/?|\?v=\/?|watch\/\?v=\/?|watch\/\?v=\/?)
1st Alternative
video.php\?v=\/?
video
matches the characters video literally (case sensitive)
. matches any character (except for line terminators)
php
matches the characters php literally (case sensitive)
\? matches the character ? with index 6310 (3F16 or 778) literally (case sensitive)
v=
matches the characters v= literally (case sensitive)
\/
matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
2nd Alternative
\?v=\/?
\? matches the character ? with index 6310 (3F16 or 778) literally (case sensitive)
v=
matches the characters v= literally (case sensitive)
\/
matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
3rd Alternative
watch\/\?v=\/?
watch
matches the characters watch literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
\? matches the character ? with index 6310 (3F16 or 778) literally (case sensitive)
v=
matches the characters v= literally (case sensitive)
\/
matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
4th Alternative
watch\/\?v=\/?
watch
matches the characters watch literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
\? matches the character ? with index 6310 (3F16 or 778) literally (case sensitive)
v=
matches the characters v= literally (case sensitive)
\/
matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
2nd Alternative
\S+\/videos\/?
\S
matches any non-whitespace character (equivalent to [^\r\n\t\f\v ])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
videos
matches the characters videos literally (case sensitive)
\/
matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
? matches the previous token between zero and one 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.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String