Regular Expressions 101

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/
^(0|[1-9]{1,3})([.,][1-9]{1,3})?$
/
gm
^ asserts position at start of a line
1st Capturing Group
(0|[1-9]{1,3})
1st Alternative
0
0 matches the character 0 with index 4810 (3016 or 608) literally (case sensitive)
2nd Alternative
[1-9]{1,3}
Match a single character present in the list below
[1-9]
{1,3} matches the previous token between 1 and 3 times, as many times as possible, giving back as needed (greedy)
1-9 matches a single character in the range between 1 (index 49) and 9 (index 57) (case sensitive)
2nd Capturing Group
([.,][1-9]{1,3})?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[.,]
.,
matches a single character in the list ., (case sensitive)
Match a single character present in the list below
[1-9]
{1,3} matches the previous token between 1 and 3 times, as many times as possible, giving back as needed (greedy)
1-9 matches a single character in the range between 1 (index 49) 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