Regular Expressions 101

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/
\s\b(?!\s)
/
gm
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
\b assert position at a word boundary: (^\w|\w$|\W\w|\w\W)
Negative Lookahead
(?!\s)
Assert that the Regex below does not match
\s matches any 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