Regular Expressions 101

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"
^\s<p>\s(?:</\w/>\s)*<a href=\"(?:(?:[a-z]:)?//(?:[^\" >/.]\.)*(?:youtube\.com|youtu\.be)[/\"])\W(?![\W\w]</(?:code|blockquote)>)(?#Separate start with youtube watch; lower TP rate, but most TP are not otherwise detected)
"
gm
^ asserts position at start of a line
\s
matches any whitespace character (equivalent to [\r\n\t\f\v  ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
<p> matches the characters <p> literally (case sensitive)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v  ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Non-capturing group
(?:</\w/>\s)*
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
< matches the character < literally (case sensitive)
/
matches the character / literally (case sensitive)
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\w
matches any word character (equivalent to [a-zA-Z0-9_])
/
matches the character / literally (case sensitive)
> matches the character > literally (case sensitive)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v  ])
<a href= matches the characters <a href= literally (case sensitive)
\" matches the character " literally (case sensitive)
Non-capturing group
(?:(?:[a-z]:)?//(?:[^\" >/.]\.)*(?:youtube\.com|youtu\.be)[/\"])
Non-capturing group
(?:[a-z]:)?
// matches the characters // literally (case sensitive)
Non-capturing group
(?:[^\" >/.]\.)*
Non-capturing group
(?:youtube\.com|youtu\.be)
Match a single character present in the list below
[/\"]
\W
matches any non-word character (equivalent to [^a-zA-Z0-9_])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Negative Lookahead
(?![\W\w]</(?:code|blockquote)>)
Assert that the Regex below does not match
Comment: Separate start with youtube watch; lower TP rate, but most TP are not otherwise detected
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

r"
"
gm

Test String