Regular Expressions 101

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/
^(([a-z0-9][a-z0-9\-]*[a-z0-9])|[a-z0-9]+\.)*([a-z]+|xn\-\-[a-z0-9]+)\.?$
/
^ asserts position at start of the string
1st Capturing Group
(([a-z0-9][a-z0-9\-]*[a-z0-9])|[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
1st Alternative
([a-z0-9][a-z0-9\-]*[a-z0-9])
2nd Capturing Group
([a-z0-9][a-z0-9\-]*[a-z0-9])
Match a single character present in the list below
[a-z0-9]
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)
Match a single character present in the list below
[a-z0-9\-]
* matches the previous token between zero 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 - with index 4510 (2D16 or 558) literally (case sensitive)
Match a single character present in the list below
[a-z0-9]
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 Alternative
[a-z0-9]+\.
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 . with index 4610 (2E16 or 568) literally (case sensitive)
3rd Capturing Group
([a-z]+|xn\-\-[a-z0-9]+)
1st Alternative
[a-z]+
Match a single character present in the list below
[a-z]
+ 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)
2nd Alternative
xn\-\-[a-z0-9]+
xn
matches the characters xn literally (case sensitive)
\- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
\- matches the character - with index 4510 (2D16 or 558) 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 . with index 4610 (2E16 or 568) literally (case sensitive)
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
$ asserts position at the end of the string, or before the line terminator right at the end of the string (if any)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/

Test String