Regular Expressions 101

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"
(?:http):\/\/(?:[^\/\r]+)(?:\/)([^\r]*)?
"
Non-capturing group
(?:http)
http
matches the characters http 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
(?:[^\/\r]+)
Match a single character not present in the list below
[^\/\r]
+ 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)
\r matches a carriage return (ASCII 13)
Non-capturing group
(?:\/)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
1st Capturing Group
([^\r]*)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
Match a single character not present in the list below
[^\r]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
\r matches a carriage return (ASCII 13)
Your regular expression does not match the subject string.

Regular Expression
No Match

r"
"

Test String