Regular Expressions 101

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/
^|\D(\d{6})\D|$
/
gm
1st Alternative
^
^ asserts position at start of a line
2nd Alternative
\D(\d{6})\D
\D matches any character that's not a digit (equivalent to [^0-9])
1st Capturing Group
(\d{6})
\d
matches a digit (equivalent to [0-9])
{6} matches the previous token exactly 6 times
\D matches any character that's not a digit (equivalent to [^0-9])
3rd 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