Find RegExp beginning

Useful, when you need to to detect what could be the first character which will pass regular expression (i.e. for lexer optimizations)

How to install

Package is available as find-regexp-beginning in NPM, so you can use it in your project using npm install find-regexp-beginning or yarn add find-regexp-beginning

What are requirements?

Code itself is written in ES6 and should work in Node.js 4+ environment (best in Node.js 9+). If you would like to use it in browser or older development, there is also transpiled and bundled (UMD) version included. You can use find-regexp-beginning/browser in your requires or FindRegexpBeginning in global environment (in browser):

// Load library
const findRegexpBeginning = require('find-regexp-beginning/browser')
 
const result = findRegexpBeginning(/abc/i) // [ 'a', 'A' ]

How to use it?

Most importantly first argument is RegExp instance.

const findRegexpBeginning = require('find-regexp-beginning')
 
const result1 = findRegexpBeginning(/abc/) // [ 'a' ]
const result2 = findRegexpBeginning(/abc/i) // [ 'a', 'A' ]

This function returns either list of possible characters on beginning or null (when there is negation or any object is accepted).

Additionally, there is second argument maxCharacters (default: 15). It's important when you want to not detect characters if they exceed number of maximum characters, i.e.:

const findRegexpBeginning = require('find-regexp-beginning')
 
const result1 = findRegexpBeginning(/[1-9]/) // [ '1', '2', '3', '4', '5', '6', '7', '8', '9' ]
const result2 = findRegexpBeginning(/[1-9]/, 9) // [ '1', '2', '3', '4', '5', '6', '7', '8', '9' ]
const result3 = findRegexpBeginning(/[1-9]/, 100) // [ '1', '2', '3', '4', '5', '6', '7', '8', '9' ]
const result4 = findRegexpBeginning(/[1-9]/, Infinity) // [ '1', '2', '3', '4', '5', '6', '7', '8', '9' ]
const result5 = findRegexpBeginning(/[1-9]/, 3) // null