Regular Expressions 101

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/
^(?:25[0-5]|2[0-4]\d|[0-1]?\d{1,2})(?:\.(?:25[0-5]|2[0-4]\d|[0-1]?\d{1,2})){3}$
/
^ asserts position at start of the string
Non-capturing group
(?:25[0-5]|2[0-4]\d|[0-1]?\d{1,2})
1st Alternative
25[0-5]
25
matches the characters 25 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)
2nd Alternative
2[0-4]\d
2 matches the character 2 with index 5010 (3216 or 628) literally (case sensitive)
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])
3rd Alternative
[0-1]?\d{1,2}
Match a single character present in the list below
[0-1]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
0-1 matches a single character in the range between 0 (index 48) and 1 (index 49) (case sensitive)
\d
matches a digit (equivalent to [0-9])
{1,2} matches the previous token between 1 and 2 times, as many times as possible, giving back as needed (greedy)
Non-capturing group
(?:\.(?:25[0-5]|2[0-4]\d|[0-1]?\d{1,2})){3}
{3} matches the previous token exactly 3 times
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
Non-capturing group
(?:25[0-5]|2[0-4]\d|[0-1]?\d{1,2})
1st Alternative
25[0-5]
25
matches the characters 25 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)
2nd Alternative
2[0-4]\d
2 matches the character 2 with index 5010 (3216 or 628) literally (case sensitive)
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])
3rd Alternative
[0-1]?\d{1,2}
Match a single character present in the list below
[0-1]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
0-1 matches a single character in the range between 0 (index 48) and 1 (index 49) (case sensitive)
\d
matches a digit (equivalent to [0-9])
{1,2} matches the previous token between 1 and 2 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