Regular Expressions 101

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/
([A-z])((?![A-z]{2,3}$)\w){2,3}
/
gm
1st Capturing Group
([A-z])
Match a single character present in the list below
[A-z]
A-z matches a single character in the range between A (index 65) and z (index 122) (case sensitive)
2nd Capturing Group
((?![A-z]{2,3}$)\w){2,3}
{2,3} matches the previous token between 2 and 3 times, as many times as possible, giving back as needed (greedy)
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
Negative Lookahead
(?![A-z]{2,3}$)
Assert that the Regex below does not match
Match a single character present in the list below
[A-z]
{2,3} matches the previous token between 2 and 3 times, as many times as possible, giving back as needed (greedy)
A-z matches a single character in the range between A (index 65) and z (index 122) (case sensitive)
$ asserts position at the end of a line
\w matches any word character (equivalent to [a-zA-Z0-9_])
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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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