Regular Expressions 101

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/
(?<=\d)DEC(?=[A-Z])
/
gm
Positive Lookbehind
(?<=\d)
Assert that the Regex below matches
\d matches a digit (equivalent to [0-9])
DEC
matches the characters DEC literally (case sensitive)
Positive Lookahead
(?=[A-Z])
Assert that the Regex below matches
Match a single character present in the list below
[A-Z]
A-Z matches a single character in the range between A (index 65) and Z (index 90) (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

Substitution

Processing...