Regular Expressions 101

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/
(?:^|\D)(\d)(?:\D|$)
/
gm
Non-capturing group
(?:^|\D)
1st Alternative
^
^ asserts position at start of a line
2nd Alternative
\D
\D matches any character that's not a digit (equivalent to [^0-9])
1st Capturing Group
(\d)
\d matches a digit (equivalent to [0-9])
Non-capturing group
(?:\D|$)
1st Alternative
\D
\D matches any character that's not a digit (equivalent to [^0-9])
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)
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