Regular Expressions 101

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/
^(?:(?:\d|[1-9]\d|1\d\d|2[0-4]\d|25[0-5])\.){3}(?:\d|[1-9]\d|1\d\d|2[0-4]\d|25[0-5])$
/
mg
^ asserts position at start of a line
Non-capturing group
(?:(?:\d|[1-9]\d|1\d\d|2[0-4]\d|25[0-5])\.){3}
{3} matches the previous token exactly 3 times
Non-capturing group
(?:\d|[1-9]\d|1\d\d|2[0-4]\d|25[0-5])
1st Alternative
\d
\d matches a digit (equivalent to [0-9])
2nd Alternative
[1-9]\d
Match a single character present in the list below
[1-9]
1-9 matches a single character in the range between 1 (index 49) and 9 (index 57) (case sensitive)
\d matches a digit (equivalent to [0-9])
3rd Alternative
1\d\d
1 matches the character 1 with index 4910 (3116 or 618) literally (case sensitive)
\d matches a digit (equivalent to [0-9])
\d matches a digit (equivalent to [0-9])
4th 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])
5th 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)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
Non-capturing group
(?:\d|[1-9]\d|1\d\d|2[0-4]\d|25[0-5])
1st Alternative
\d
\d matches a digit (equivalent to [0-9])
2nd Alternative
[1-9]\d
Match a single character present in the list below
[1-9]
1-9 matches a single character in the range between 1 (index 49) and 9 (index 57) (case sensitive)
\d matches a digit (equivalent to [0-9])
3rd Alternative
1\d\d
1 matches the character 1 with index 4910 (3116 or 618) literally (case sensitive)
\d matches a digit (equivalent to [0-9])
\d matches a digit (equivalent to [0-9])
4th 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])
5th 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)
$ asserts position at the end of a line
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

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