Regular Expressions 101

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/
(?:^|(?<=\s))@(?!\s)
/
gim
Non-capturing group
(?:^|(?<=\s))
1st Alternative
^
^ asserts position at start of a line
2nd Alternative
(?<=\s)
Positive Lookbehind
(?<=\s)
Assert that the Regex below matches
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
@ matches the character @ with index 6410 (4016 or 1008) literally (case insensitive)
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)
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
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

/
/
gim

Test String

Substitution

Processing...