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date:: [[2023-05-08]]
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# [[Regular expressions in js]]
[[Regular Expressions]] in [[JavaScript]] are used to search for substrings within a string when you don't know the exact values and instead just know the pattern.
## Defining the regular expression
The regular expression is the pattern you're looking for, so start by describing that. There are a whole bunch of symbols to learn, [so here's a cheat sheet I found](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_expressions/Cheatsheet).
For example:
```javascript
const regex = /S(\d+)/g;
```
The line above is defining a pattern that would match with something like this: `S83`.
## Finding what matches: `match()`
`match()` is the method that looks for something in the string that matches the regular expression.
```javascript
const filepath = "Wildemount/_Tues Wildemount DM Notes/Previous Session Notes/S83 2023-Etwas"
const regex = /S(\d+)/g;
const found = filepath.match(regex);
let lastGameNum = found[0].replace("S",""); // 83
```
`match()` returns an [[Iterating through elements in an array in js|array]] of all the elements that matched the regular expression. In the example above, `found[0]` is `S83`.
## Finding the index of what matches: `search()`
```javascript
const filepath = "Wildemount/_Tues Wildemount DM Notes/Previous Session Notes/S83 2023-Etwas"
const regex = /S(\d+)/g;
const foundIndex = filepath.search(regex); // 60
```