Regular Expressions 101

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/
([abc])([de])\2\1
/
g
1st Capturing Group
([abc])
Match a single character present in the list below
[abc]
abc
matches a single character in the list abc (case sensitive)
2nd Capturing Group
([de])
Match a single character present in the list below
[de]
de
matches a single character in the list de (case sensitive)
\2 matches the same text as most recently matched by the 2nd capturing group
\1 matches the same text as most recently matched by the 1st capturing group
Global pattern flags
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

/
/
g

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