Regular Expressions 101

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/
^(http:\/\/www\.|https:\/\/www\.|http:\/\/|https:\/\/)?[a-z0-9]([\-\.][a-z0-9])*\.[a-z](:[0-9])?(\/.)?$
/
mg
^ asserts position at start of a line
1st Capturing Group
(http:\/\/www\.|https:\/\/www\.|http:\/\/|https:\/\/)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
1st Alternative
http:\/\/www\.
http: matches the characters http: literally (case sensitive)
\/ matches the character / literally (case sensitive)
\/ matches the character / literally (case sensitive)
www matches the characters www literally (case sensitive)
\. matches the character . literally (case sensitive)
2nd Alternative
https:\/\/www\.
https: matches the characters https: literally (case sensitive)
\/ matches the character / literally (case sensitive)
\/ matches the character / literally (case sensitive)
www matches the characters www literally (case sensitive)
\. matches the character . literally (case sensitive)
3rd Alternative
http:\/\/
http: matches the characters http: literally (case sensitive)
\/ matches the character / literally (case sensitive)
\/ matches the character / literally (case sensitive)
4th Alternative
https:\/\/
https: matches the characters https: literally (case sensitive)
\/ matches the character / literally (case sensitive)
\/ matches the character / literally (case sensitive)
Match a single character present in the list below
[a-z0-9]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
2nd Capturing Group
([\-\.][a-z0-9])*
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
Match a single character present in the list below
[\-\.]
{1} matches the previous token exactly one time (meaningless quantifier)
\- matches the character - literally (case sensitive)
\. matches the character . literally (case sensitive)
Match a single character present in the list below
[a-z0-9]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
\. matches the character . literally (case sensitive)
Match a single character present in the list below
[a-z]
{2,5} matches the previous token between 2 and 5 times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
3rd Capturing Group
(:[0-9])?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
: matches the character : literally (case sensitive)
Match a single character present in the list below
[0-9]
4th Capturing Group
(\/.)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\/ matches the character / literally (case sensitive)
.
matches any character (except for line terminators)
$ asserts position at the end of a line
Global pattern flags
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.

Regular Expression
No Match

/
/
mg

Test String