Regular Expressions 101

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/
^((?!-([ABC])(\d{3})).)*$
/
gm
^ asserts position at start of a line
1st Capturing Group
((?!-([ABC])(\d{3})).)*
* matches the previous token between zero 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
Negative Lookahead
(?!-([ABC])(\d{3}))
Assert that the Regex below does not match
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
2nd Capturing Group
([ABC])
Match a single character present in the list below
[ABC]
ABC
matches a single character in the list ABC (case sensitive)
3rd Capturing Group
(\d{3})
\d
matches a digit (equivalent to [0-9])
{3} matches the previous token exactly 3 times
. matches any character (except for line terminators)
$ 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