Regular Expressions 101

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/
^[^#]*#([^#]*)#[^#]*#\1.*$
/
gim
^ asserts position at start of a line
Match a single character not present in the list below
[^#]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
# matches the character # with index 3510 (2316 or 438) literally (case insensitive)
# matches the character # with index 3510 (2316 or 438) literally (case insensitive)
1st Capturing Group
([^#]*)
Match a single character not present in the list below
[^#]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
# matches the character # with index 3510 (2316 or 438) literally (case insensitive)
# matches the character # with index 3510 (2316 or 438) literally (case insensitive)
Match a single character not present in the list below
[^#]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
# matches the character # with index 3510 (2316 or 438) literally (case insensitive)
# matches the character # with index 3510 (2316 or 438) literally (case insensitive)
\1 matches the same text as most recently matched by the 1st capturing group
.
matches any character (except for line terminators)
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
$ asserts position at the end of a line
Global pattern flags
g modifier: global. All matches (don't return after first match)
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
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

/
/
gim

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