Can you use a RegEx in javascript to see if a string contains a certain number of certain characters?

Question

Say I am given a string (555)-555-5555, is there a way to check if the string contains exactly 10 occurrences of the number 5? I can think of other ways to solve this, but I am curious if this can be done with a regEx. I tried using var re = new regExp(/5{10}/); but apparently this only matches ten 5's in a row.

Answer 1

1) Delete all non-5, count the remains.

str.replace(/[^5]/g, '').length == 10

2) Count 5-with-a-tail groups:

str.match(/^[^5]*(?:5[^5]*){10}$/)

EDIT: fixed bugs. Also, the explanation: the entirety of the string (^...$) consists of any number of non-5, followed by exactly 10 groups of one 5 and any number of non-5. Thus,

x55pp5peep5o5555taco5lol5

breaks down into

^               start of string
[^5]*           x
(?:5[^5]*){10}  5
                5pp
                5peep
                5o
                5
                5
                5
                5taco
                5lol
                5
$               end of string

Answer 2

A "mostly" regex solution would be to match replace all other characters with the empty string and check the length of your result. Or similarly you could match and replace the character you want with the empty string and take the difference and see if it's the number you want.