Match 10 digit alternating numbers-CodePudding

I want a regex which matches 10 digit alternating numbers like 1212121212 3434343434 so on. I know this is not right way to ask here. But I tried a lot from my side I was not able to figure out so reaching out for help.

It cannot be 1313131313. I should be the next number.

CodePudding user response：

If the string is always 10 characters long and we always have single-digit numbers then simply take the first two characters, repeat 5 times and compare:

const check = str => {
  const a =  str[0];
  const b =  str[1];
  return a 1 === b && `${a}${b}`.repeat(5) === str;
};

check('1212121212');
//=> true

check('1212121215');
//=> false

check('1313131313');
//=> false

CodePudding user response：

If the repetition is always two figures you can do something like (\d\d)\1{4}.

This is create a group with two digits (\d\d), find the same match as previous four times in a row \1{4}.

CodePudding user response：

You might also use Array.from and specify a length and an offset to get the alternation for the next number:

const alternating = (len, offset) =>
  Array.from({ length: len }, (_, index) => (index % 2)   offset).join('');

console.log(alternating(10, 1) === "1212121212");
console.log(alternating(10, 3) === "3434343434");
console.log(alternating(10, 2) === "1313131313");

CodePudding user response：

For pure regex you would just have to work out the different combinations because regex doesn't support the concept of numbers nor their sequence; everything is a string of chars.

^(01|12|23|34|45|56|67|78|89)\1{4}$

If 9090909090 is valid then:

^(01|12|23|34|45|56|67|78|89|90)\1{4}$