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^(([a-z0-9][a-z0-9\-]*[a-z0-9])|[a-z0-9]+\.)*([a-z]+|xn\-\-[a-z0-9]+)\.?$

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^ 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 45_{10} (2D_{16} or 55_{8}) 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 46_{10} (2E_{16} or 56_{8}) 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 45_{10} (2D_{16} or 55_{8}) literally (case sensitive)

\- matches the character - with index 45_{10} (2D_{16} or 55_{8}) 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 46? 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.

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