Regular Expressions 101

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/
^\d([-]|\s)?(\d)?$
/
gm
^ asserts position at start of a line
\d
matches a digit (equivalent to [0-9])
{5} matches the previous token exactly 5 times
1st Capturing Group
([-]|\s)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
1st Alternative
[-]
Match a single character present in the list below
[-]
- matches the character - literally (case sensitive)
This hyphen is treated literally, which might be confusing for others. Consider escaping it or placing at the start or end of the class!
2nd Alternative
\s
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
2nd Capturing Group
(\d)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\d
matches a digit (equivalent to [0-9])
{4} matches the previous token exactly 4 times
$ 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