Regular Expressions 101

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/
^((?:1?\d?)?\d|2(?:[0-4]\d|5[0-5]))(?:\.(?1)){3}$
/
gm
^ asserts position at start of a line
1st Capturing Group
((?:1?\d?)?\d|2(?:[0-4]\d|5[0-5]))
1st Alternative
(?:1?\d?)?\d
Non-capturing group
(?:1?\d?)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
1
matches the character 1 with index 4910 (3116 or 618) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\d matches a digit (equivalent to [0-9])
2nd Alternative
2(?:[0-4]\d|5[0-5])
2 matches the character 2 with index 5010 (3216 or 628) literally (case sensitive)
Non-capturing group
(?:[0-4]\d|5[0-5])
1st Alternative
[0-4]\d
Match a single character present in the list below
[0-4]
0-4 matches a single character in the range between 0 (index 48) and 4 (index 52) (case sensitive)
\d matches a digit (equivalent to [0-9])
2nd Alternative
5[0-5]
5 matches the character 5 with index 5310 (3516 or 658) literally (case sensitive)
Match a single character present in the list below
[0-5]
0-5 matches a single character in the range between 0 (index 48) and 5 (index 53) (case sensitive)
Non-capturing group
(?:\.(?1)){3}
{3} matches the previous token exactly 3 times
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
(?1) recurses the 1st subpattern
$ asserts position at the end of a line
Global pattern flags
g modifier: global. All matches (don't return after first match)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String