Regular Expressions 101

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/
(^9+((\.9+)+)?$)?
/
gm
1st Capturing Group
(^9+((\.9+)+)?$)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
^ asserts position at start of a line
9
matches the character 9 with index 5710 (3916 or 718) literally (case sensitive)
2nd Capturing Group
((\.9+)+)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
3rd Capturing Group
(\.9+)+
+ matches the previous token between one and unlimited 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
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
9
matches the character 9 with index 5710 (3916 or 718) literally (case sensitive)
$ 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)
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