Regular Expressions 101

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/
(?<!\S)[-+]\d\.\d(?:[eE][-+]\d)?(?!\S)
/
gm
Negative Lookbehind
(?<!\S)
Assert that the Regex below does not match
\S matches any non-whitespace character (equivalent to [^\r\n\t\f\v ])
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 - 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!
+ matches the character + literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
\.
matches the character . literally (case sensitive)
? 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 one and unlimited times, as many times as possible, giving back as needed (greedy)
Non-capturing group
(?:[eE][-+]\d)?
? 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
[eE]
eE matches a single character in the list eE (case sensitive)
Match a single character present in the list below
[-+]
\d
matches a digit (equivalent to [0-9])
Negative Lookahead
(?!\S)
Assert that the Regex below does not match
\S matches any non-whitespace character (equivalent to [^\r\n\t\f\v ])
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