Regular Expressions 101

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/
\d(?=(?:[* \d]\d){4}$)
/
gm
\d matches a digit (equivalent to [0-9])
Positive Lookahead
(?=(?:[* \d]\d){4}$)
Assert that the Regex below matches
Non-capturing group
(?:[* \d]\d){4}
{4} matches the previous token exactly 4 times
Match a single character present in the list below
[ \d]
* 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)
\d matches a digit (equivalent to [0-9])
\d matches a digit (equivalent to [0-9])
$ asserts position at the end of a line
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.

Regular Expression
No Match

/
/
gm

Test String

Substitution

Processing...