regexr

Easily compose regular expressions. Doing this with plain strings would otherwise be tedious and error prone due to having to double-escape backslashes.

npm install regexr

Basic example:

import r from 'regexr'

const int = /\d+/
const USD = r`\$${int}(\.${int})?` // f.e. $3.45 or $5

(Note that int is an instance of RegExp and can be composed into the template string, and the resulting USD is also a RegExp)

Regexr provides an ES6 template tag function that makes it easy to compose RegExps using template strings without double-escaped hell.

In ES5 and below, we may try to compose the regular expressions like so:

const int = '\d+'
let USD = new RegExp('\$'+int+'(\.'+int+')?`) // this won't work!

but if you're experienced enough, you'd know that if you want to compose regular expressions using ES5 strings you have to escape the escape:

const int = '\\d+'
const USD = new RegExp('\\$'+int+'(\\.'+int+')?`) // correct!

Imagine making more complex regexes! For example, compare the following two examples achieving the same thing in ES5 and ES6 respectively:

// in ES5, the double escaping can get confusing:
const spaceRegex = '\\s*'
const finalRegex = new RegExp('\\(' + spaceRegex + '\\/\\[\\\\\\d+\\]\\)*$', 'g')
console.log(!!'( /[\\12358])'.match(finalRegex)) // true

// in ES6, we don't have to double escape, thanks to regexr:
import r from 'regexr'

const spaceRegex = r`\s*`
const finalRegex = r`/\(${spaceRegex}\/\[\\\d+\]\)*$/g`
console.log(!!'( /[\\12358])'.match(finalRegex)) // true

API

r`` template tag function

import r from 'regexr'
// or
const r = require('regexr').default

r`` is a template tag function that converts the given string into a RegExp without requiring double escaping. Instances of RegExp can be mixed into the string, and will be composed into the final RegExp.

Example:

const digit = /\d/
const integer = r`/${digit}+/`
const number = r`/${integer}|${digit}*\.${integer}|${integer}\.${digit}*/` // f.e. 4.2, .5, 5.

Helpers

escape (alias e)

Escape (add backslashes to) a string for so that we can match all symbols in the string literally when the string is used as a regex.

In the following example, we want to find occurrences of the string "value: $5.00" in some input, so we need to escape the money string so that the dollar symbol ($) doesn't represent end-of-line and the period (.) doesn't mean any character:

import {e} from 'regexr'

const money = '$5.00'
const fiveDollarRegex = r`value: ${e(money)}`

console.log(fiveDollarRegex) // /value: \$5\.00/
console.log('value: $5.00'.match(fiveDollarRegex)) // true
console.log('value: $5.50'.match(fiveDollarRegex)) // false

Hand-picked Regexes

Regexr comes with some pre-selected regular expressions. For example, we can rewrite the first example:

import r from 'regexr'
import * as regs from 'regexr/regexes'

const USD = r`\$${regs.integer}(\.${regs.integer})?` // f.e. $3.45 or $5

where regs.integer is an instance of RegExp.

NOTE! Some of the following RegExps require to be wrapped in () when they are being composed into bigger RegExps. These will be noted below.

regs.identifier

Matches a valid JavaScript identifier. See this for details.

Requires wrapping in () when being composed.

For example, to match a the beginning of a JS variable declaration, you could write:

import * as regs from 'regexr/regexes'
const variableDeclaration = r`(const|let|var)\s+(${regs.identifier})\s*=`
!!'const foo  ='.match(variableDeclaration) // true
!!'const foo bar ='.match(variableDeclaration) // false

regs.digit

Matches a single numerical digit (0-9).

Example:

import * as regs from 'regexr/regexes'
!!' 8 '.match(r` ${regs.digit} `) // true
!!' 25 '.match(r` ${regs.digit} `) // false

regs.integer

Matches 1 or more digits.

Example:

import * as regs from 'regexr/regexes'
!!' 432 '.match(r` ${regs.integer} `) // true

regs.number

Matches a JavaScript Number.

Example:

import * as regs from 'regexr/regexes'
!!'3'.match(regs.number) // true
!!'432'.match(regs.number) // true
!!'4.2'.match(regs.number) // true
!!'5.'.match(regs.number) // true
!!'.34'.match(regs.number) // true

regs.identifierList

Matches a comma separated list of legal JavaScript identifiers.

Example:

import * as regs from 'regexr/regexes'
const identifiersInsideParens = r`\(${regs.identifierList}\)`

!!'(foo,  bar,baz)'.match(identifiersInsideParens) // true
!!'(foo, ,bar, baz)'.match(identifiersInsideParens) // false

regs.functionHeader

Matches a JavaScript function header.

Example:

import * as regs from 'regexr/regexes'
const identifiersInsideParens = r`\(${regs.identifierList}\)`

!!'function() {'.match(regs.functionHeader) // true
!!'function asdf() {'.match(regs.functionHeader) // true
!!'function (asdf ) {'.match(regs.functionHeader) // true
!!'function asdf (asdf ) {'.match(regs.functionHeader) // true
!!'function asdf(asdf  , asdf, ) {'.match(regs.functionHeader) // true
!!'function (asdf, asdf, asdfa asdf ) {'.match(regs.functionHeader) // false
!!'function asdf (asdf, asdf, asdfa asdf ) {'.match(regs.functionHeader) // false
!!'function asdf asdf (asdf, asdf, asdfa ) {'.match(regs.functionHeader) // false
!!'function asdf asdf (, asdf, asdf,) {'.match(regs.functionHeader) // false
!!'function (asdf asdf) {'.match(regs.functionHeader) // false
!!'function (asdf,,) {'.match(regs.functionHeader) // false