Regular expression to count number of commas in a string

Question

How can I build a regular expression that will match a string of any length containing any characters but which must contain 21 commas?

Answer 1

/^([^,]*,){21}[^,]*$/

That is:

^     Start of string
(     Start of group
[^,]* Any character except comma, zero or more times
,     A comma
){21} End and repeat the group 21 times
[^,]* Any character except comma, zero or more times again
$     End of string