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

If you're using a regex variety that supports the Possessive quantifier (e.g. Java), you can do:

^(?:[^,]*+,){21}[^,]*+$

The Possessive quantifier can be better performance than a Greedy quantifier.

Explanation:

(?x)    # enables comments, so this whole block can be used in a regex.
^       # start of string
(?:     # start non-capturing group
[^,]*+  # as many non-commas as possible, but none required
,       # a comma
)       # end non-capturing group
{21}    # 21 of previous entity (i.e. the group)
[^,]*+  # as many non-commas as possible, but none required
$       # end of string