Regular Expressions 101

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/
^[0-9]{0,9}(-[0-9]{0,4})?$
/
gm
^ asserts position at start of a line
Match a single character present in the list below
[0-9]
{0,9} matches the previous token between 0 and 9 times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
1st Capturing Group
(-[0-9]{0,4})?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
Match a single character present in the list below
[0-9]
{0,4} matches the previous token between 0 and 4 times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (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