Regular Expressions 101

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^[+-]?((0\.\d+)|([1-9]+(\d?\.\d+)?))$
/
^ asserts position at start of the string
Match a single character present in the list below
[+-]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
+ matches the character + with index 4310 (2B16 or 538) literally (case sensitive)
- matches the character - with index 4510 (2D16 or 558) 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!
1st Capturing Group
((0\.\d+)|([1-9]+(\d?\.\d+)?))
1st Alternative
(0\.\d+)
2nd Capturing Group
(0\.\d+)
0 matches the character 0 with index 4810 (3016 or 608) literally (case sensitive)
\. 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
([1-9]+(\d?\.\d+)?)
3rd Capturing Group
([1-9]+(\d?\.\d+)?)
Match a single character present in the list below
[1-9]
+ matches the previous token between one and unlimited 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)
4th Capturing Group
(\d?\.\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])
? 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)
$ asserts position at the end of the string
Your regular expression does not match the subject string.

Regular Expression
No Match

/
/

Test String