Regular Expressions 101

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/
(?<!r)d
/
gm
Negative Lookbehind
(?<!r)
Assert that the Regex below does not match
r matches the character r with index 11410 (7216 or 1628) literally (case sensitive)
d matches the character d with index 10010 (6416 or 1448) literally (case sensitive)
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