Regular Expressions 101

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"
^(?:(?:25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])\.){3}(?:25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])$
"
g
^ asserts position at start of the string
Non-capturing group
(?:(?:25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])\.){3}
{3} matches the previous token exactly 3 times
Non-capturing group
(?:25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[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
1[0-9][0-9]
1 matches the character 1 with index 4910 (3116 or 618) literally (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]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
4th Alternative
[1-9]?[0-9]
Match a single character present in the list below
[1-9]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
1-9 matches a single character in the range between 1 (index 49) and 9 (index 57) (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)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
Non-capturing group
(?:25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[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
1[0-9][0-9]
1 matches the character 1 with index 4910 (3116 or 618) literally (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]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
4th Alternative
[1-9]?[0-9]
Match a single character present in the list below
[1-9]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
1-9 matches a single character in the range between 1 (index 49) and 9 (index 57) (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)
$ asserts position at the end of the string, or before the line terminator right at the end of the string (if any)
Global pattern flags
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.

Regular Expression
No Match

r"
"
g

Test String

Substitution

Processing...