Regular Expressions 101

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/
^(((25[0-5])|(2[0-4]\d)|(1\d{2})|(\d{1,2}))\.){3}(((25[0-5])|(2[0-4]\d)|(1\d{2})|(\d{1,2})))$
/
gm
^ asserts position at start of a line
1st Capturing Group
(((25[0-5])|(2[0-4]\d)|(1\d{2})|(\d{1,2}))\.){3}
{3} matches the previous token exactly 3 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]\d)|(1\d{2})|(\d{1,2}))
1st Alternative
(25[0-5])
3rd Capturing Group
(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)
4th Capturing Group
(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
(1\d{2})
5th Capturing Group
(1\d{2})
1 matches the character 1 with index 4910 (3116 or 618) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{2} matches the previous token exactly 2 times
4th Alternative
(\d{1,2})
6th Capturing Group
(\d{1,2})
\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)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
7th Capturing Group
(((25[0-5])|(2[0-4]\d)|(1\d{2})|(\d{1,2})))
8th Capturing Group
((25[0-5])|(2[0-4]\d)|(1\d{2})|(\d{1,2}))
1st Alternative
(25[0-5])
9th Capturing Group
(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)
10th Capturing Group
(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
(1\d{2})
11th Capturing Group
(1\d{2})
1 matches the character 1 with index 4910 (3116 or 618) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{2} matches the previous token exactly 2 times
4th Alternative
(\d{1,2})
12th Capturing Group
(\d{1,2})
\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 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