Regular Expressions 101

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/
(?:\G(?!^)|!)(?:[^:\n]*:)+?\K\d{6}
/
g
Non-capturing group
(?:\G(?!^)|!)
1st Alternative
\G(?!^)
\G asserts position at the end of the previous match or the start of the string for the first match
Negative Lookahead
(?!^)
Assert that the Regex below does not match
^ asserts position at start of the string
2nd Alternative
!
! matches the character ! with index 3310 (2116 or 418) literally (case sensitive)
Non-capturing group
(?:[^:\n]*:)+?
+? matches the previous token between one and unlimited times, as few times as possible, expanding as needed (lazy)
Match a single character not present in the list below
[^:\n]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
\n matches a line-feed (newline) character (ASCII 10)
: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
\K resets the starting point of the reported match. Any previously consumed characters are no longer included in the final match
\d
matches a digit (equivalent to [0-9])
{6} matches the previous token exactly 6 times
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