---
id: 5e6dd14797f5ce267c2f19d0
title: Look-and-say sequence
challengeType: 5
forumTopicId: 385277
dashedName: look-and-say-sequence
---
# --description--
The [Look and say sequence](https://en.wikipedia.org/wiki/Look and say sequence) is a recursively defined sequence of numbers.
Sequence Definition
- Take a decimal number
- Look at the number, visually grouping consecutive runs of the same digit.
- Say the number, from left to right, group by group; as how many of that digit there are - followed by the digit grouped.
This becomes the next number of the sequence.
An example:
- Starting with the number 1, you have one 1 which produces 11
- Starting with 11, you have two 1's. I.E.: 21
- Starting with 21, you have one 2, then one 1. I.E.: (12)(11) which becomes 1211
- Starting with 1211, you have one 1, one 2, then two 1's. I.E.: (11)(12)(21) which becomes 111221
# --instructions--
Write a function that accepts a string as a parameter, processes it, and returns the resultant string.
# --hints--
`lookAndSay` should be a function.
```js
assert(typeof lookAndSay == 'function');
```
`lookAndSay("1")` should return a string.
```js
assert(typeof lookAndSay('1') == 'string');
```
`lookAndSay("1")` should return `"11"`.
```js
assert.equal(lookAndSay('1'), '11');
```
`lookAndSay("11")` should return `"21"`.
```js
assert.equal(lookAndSay('11'), '21');
```
`lookAndSay("21")` should return `"1211"`.
```js
assert.equal(lookAndSay('21'), '1211');
```
`lookAndSay("1211")` should return `"111221"`.
```js
assert.equal(lookAndSay('1211'), '111221');
```
`lookAndSay("3542")` should return `"13151412"`.
```js
assert.equal(lookAndSay('3542'), '13151412');
```
# --seed--
## --seed-contents--
```js
function lookAndSay(str) {
}
```
# --solutions--
```js
function lookAndSay(str) {
return str.replace(/(.)\1*/g, function(seq, p1) {
return seq.length.toString() + p1;
});
}
```