Regular Expressions 101

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/
\b((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(\.|$)){4}\b
/
mg
\b assert position at a word boundary: (^\w|\w$|\W\w|\w\W)
1st Capturing Group
((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(\.|$)){4}
{4} matches the previous token exactly 4 times
A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
2nd Capturing Group
(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)
1st Alternative
25[0-5]
25
matches the characters 25 literally (case sensitive)
Match a single character present in the list below
[0-5]
0-5 matches a single character in the range between 0 (index 48) and 5 (index 53) (case sensitive)
2nd Alternative
2[0-4][0-9]
2 matches the character 2 with index 5010 (3216 or 628) literally (case sensitive)
Match a single character present in the list below
[0-4]
0-4 matches a single character in the range between 0 (index 48) and 4 (index 52) (case sensitive)
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
3rd Alternative
[01]?[0-9][0-9]?
Match a single character present in the list below
[01]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
01
matches a single character in the list 01 (case sensitive)
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
Match a single character present in the list below
[0-9]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
3rd Capturing Group
(\.|$)
1st Alternative
\.
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
2nd Alternative
$
$ asserts position at the end of a line
\b assert position at a word boundary: (^\w|\w$|\W\w|\w\W)
Global pattern flags
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
mg

Test String