Exact number of times a given character must repeat Regex

Question

Hello, I’m with an exercise in formal languages, of which we work with Regex, the exercise consists in validating a chain that has exactly four 1s, for example:

chain 1: 0110101000 - valid;

jail 2: 010110 - invalid;

chain 3: 011011011 - invalid;

What I’ve been able to do so far is validate a string with four or more 1s, but I haven’t been able to determine exactly the number of times a given character should repeat:

Follows the expression I’ve managed to develop so far: /(\S*1){4}.*/

This expression validates strings with more than four 1s, I need to validate a string that has exactly four 1s.

Thanks in advance!!

Answer 1

I got it with the following regex:

/^(0*10*){4}$/

https://regex101.com/r/sGa5mR/1

In formal languages, the use of 0s and 1s. Otherwise I would enjoy my regex too.

Another way for you to do the same thing without frufruzentos quantifiers is like this:

/^0*10*10*10*10*$/

https://regex101.com/r/fYzNOc/1

Answer 2

^{The other characters can only be zeros, or they can be anything other than 1?}

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Anyway, I’m using the expression [^1] (everything that is not 1) together with ^ (string start) and $ (end of string), to indicate that regex has to check the whole string, from start to finish:

/^([^1]*1[^1]*){4}$/

Explaining:

• [^1]* - zero or more occurrences of anything other than 1 (to check what comes before the first 1)
• 1 - own 1
• [^1]* - zero or more occurrences of anything other than 1 (again, but here is to check what comes after the last 1)
• The quantifier {4} says the previous expression (everything in parentheses: ([^1]*1[^1]*)) can only be repeated four times

Watch it work here.

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Another option (since the question has the tag javascript) is to count the number of occurrences of the character 1, using match:

// conta a quantidade de "1" na string
var quantidadeDeUm = ("01001001001".match(/1/g) || []).length;

console.log(quantidadeDeUm); // 4

Then just check if the quantity is 4. The advantage is that the regex is much simpler, and in my opinion, it is a much easier code to understand and maintain.

The excerpt || [] returns an empty array, in case the match do not find any occurrence. I did this because when there is no occurrence, the match returns null, then in this case I use an empty array not to give a Typeerror: null.