Regular Expressions 101

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/
^([+-])?\d(?:\.\d)?(?(1)%)?$
/
gm
^ asserts position at start of a line
1st Capturing Group
([+-])?
? 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)
\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 . 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)
Conditional
(?(1)%)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
Conditionally matches one of two options depending on whether the 1st capturing group matched
If condition is met, match the following regex
%
% matches the character % literally (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