Regular Expressions 101

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/
^([0,1][0-9]|2[0-3]):([0-5][0-9])$
/
gm
^ asserts position at start of a line
1st Capturing Group
([0,1][0-9]|2[0-3])
1st Alternative
[0,1][0-9]
Match a single character present in the list below
[0,1]
0,1
matches a single character in the list 0,1 (case sensitive)
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
2nd Alternative
2[0-3]
2 matches the character 2 with index 5010 (3216 or 628) literally (case sensitive)
Match a single character present in the list below
[0-3]
0-3 matches a single character in the range between 0 (index 48) and 3 (index 51) (case sensitive)
: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
2nd Capturing Group
([0-5][0-9])
Match a single character present in the list below
[0-5]
0-5 matches a single character in the range between 0 (index 48) and 5 (index 53) (case sensitive)
Match a single character present in the list below
[0-9]
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.

Regular Expression
No Match

/
/
gm

Test String