Regular Expressions 101

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/
^(-?(?:(?=[1-9]|0(?!\d))\d+(?:\.\d+)?)|\.\d+)$
/
gm
^ asserts position at start of a line
1st Capturing Group
(-?(?:(?=[1-9]|0(?!\d))\d+(?:\.\d+)?)|\.\d+)
1st Alternative
-?(?:(?=[1-9]|0(?!\d))\d+(?:\.\d+)?)
-
matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
Non-capturing group
(?:(?=[1-9]|0(?!\d))\d+(?:\.\d+)?)
Positive Lookahead
(?=[1-9]|0(?!\d))
Assert that the Regex below matches
1st Alternative
[1-9]
Match a single character present in the list below
[1-9]
1-9 matches a single character in the range between 1 (index 49) and 9 (index 57) (case sensitive)
2nd Alternative
0(?!\d)
0 matches the character 0 with index 4810 (3016 or 608) literally (case sensitive)
Negative Lookahead
(?!\d)
Assert that the Regex below does not match
\d matches a digit (equivalent to [0-9])
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
Non-capturing group
(?:\.\d+)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
2nd Alternative
\.\d+
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
$ 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