Regular Expressions 101

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/
^((?&byte))\.((?&byte))\.((?&byte))\.((?&byte))$(?(DEFINE)(?<byte>25[0-5]|2[0-4]\d|[01]?\d\d?))
/
gm
^ asserts position at start of a line
1st Capturing Group
((?&byte))
(?&byte) recurses the subpattern named byte
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
2nd Capturing Group
((?&byte))
(?&byte) recurses the subpattern named byte
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
3rd Capturing Group
((?&byte))
(?&byte) recurses the subpattern named byte
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
4th Capturing Group
((?&byte))
(?&byte) recurses the subpattern named byte
$ asserts position at the end of a line
Subpattern definition construct
(?(DEFINE)(?<byte>25[0-5]|2[0-4]\d|[01]?\d\d?))
May only be used to define functions. No matching is done in this group.
Named Capture Group byte
(?<byte>25[0-5]|2[0-4]\d|[01]?\d\d?)
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
[01]?\d\d?
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)
\d matches a digit (equivalent to [0-9])
\d
matches a digit (equivalent to [0-9])
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
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

Code Generator

Generated Code

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For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php