Regular Expressions 101

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/
^.(?=\d[a-z]\d[a-z]|$)
/
gmi
^ asserts position at start of a line
.
matches any character (except for line terminators)
*? matches the previous token between zero and unlimited times, as few times as possible, expanding as needed (lazy)
Positive Lookahead
(?=\d[a-z]\d[a-z]|$)
Assert that the Regex below matches
1st Alternative
\d[a-z]\d[a-z]
\d
matches a digit (equivalent to [0-9])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[a-z]
a-z matches a single character in the range between a (index 97) and z (index 122) (case insensitive)
\d
matches a digit (equivalent to [0-9])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[a-z]
a-z matches a single character in the range between a (index 97) and z (index 122) (case insensitive)
2nd Alternative
$
$ 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)
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gmi

Test String

Substitution

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Code Generator

Generated Code

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Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report.
For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php