Regular Expressions 101

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^((([0-9A-Fa-f]{1,4}:){1,6}:)|(([0-9A-Fa-f]{1,4}:){7}))([0-9A-Fa-f]{1,4})$
/
^ asserts position at start of the string
1st Capturing Group
((([0-9A-Fa-f]{1,4}:){1,6}:)|(([0-9A-Fa-f]{1,4}:){7}))
1st Alternative
(([0-9A-Fa-f]{1,4}:){1,6}:)
2nd Capturing Group
(([0-9A-Fa-f]{1,4}:){1,6}:)
3rd Capturing Group
([0-9A-Fa-f]{1,4}:){1,6}
{1,6} matches the previous token between 1 and 6 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
[0-9A-Fa-f]
{1,4} matches the previous token between 1 and 4 times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
A-F matches a single character in the range between A (index 65) and F (index 70) (case sensitive)
a-f matches a single character in the range between a (index 97) and f (index 102) (case sensitive)
: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
2nd Alternative
(([0-9A-Fa-f]{1,4}:){7})
4th Capturing Group
(([0-9A-Fa-f]{1,4}:){7})
5th Capturing Group
([0-9A-Fa-f]{1,4}:){7}
{7} matches the previous token exactly 7 times
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
[0-9A-Fa-f]
{1,4} matches the previous token between 1 and 4 times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
A-F matches a single character in the range between A (index 65) and F (index 70) (case sensitive)
a-f matches a single character in the range between a (index 97) and f (index 102) (case sensitive)
: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
6th Capturing Group
([0-9A-Fa-f]{1,4})
Match a single character present in the list below
[0-9A-Fa-f]
{1,4} matches the previous token between 1 and 4 times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
A-F matches a single character in the range between A (index 65) and F (index 70) (case sensitive)
a-f matches a single character in the range between a (index 97) and f (index 102) (case sensitive)
$ 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