escape-regex-string

escape-regex-string

Escapes a string literal for use as an argument in the standard RegExp constructor.

Interface

escape-regex-string

(require('escape-regex-string'))(unescapedString[, escapeCharsRegex])

• unescapedString String
  • The regular expression pattern string in which all special characters need to be escaped.
• escapeCharsRegex RegExp
  • Defaults to value of member escape-regex-string.defaultEscapeCharsRegex (see below)

Returns the passed unescapedString with all RegExp special characters escaped.

escape-regex-string.defaultEscapeCharsRegex

(require('escape-regex-string')).defaultEscapeCharsRegex

A read-only RegExp instance containing the default pattern used to escape passed strings. If a RegExp instance is manually passed into a function call to this module, the passed RegExp value will be used instead of this default value.

Example Usage

let escapeRegexString = require('escape-regex-string');
let regexPatternString = '$&*()awsd';
let escapedRegexPatternString = escapeRegexString(regexPatternString);
console.log(escapedRegexPatternString); // '\\$&\\*\\(\\)awsd'
let regExpObject = new RegExp(escapedRegexPatternString);
console.log(regExpObject); // /\$&\*\(\)awsd/

Explanation

The built-in JavaScript RegExp object's constructor accepts a regular expression pattern in the form of another RegExp instance or a string. And, as we know, the JavaScript regular expression syntax includes a set of characters with special meaning when interpreted by the regex engine, such as the caret (^) and the asterisk (*). A problem is introduced when one attempts to create a regular expression pattern in a string (not a RegExp literal), for use in the RegExp constructor, that contains special regular expression characters to be matched literally. Consider the following extra-verbose example:

A developer wants to use a RegExp to match caret characters in a given string:

let inputStr = 'a^2';

For whatever reason, the dev is unwilling or unable to use the RegExp literal syntax and therefore attempts to define a regular expression pattern in a string to be passed to the RegExp constructor:

let firstPattern = '^';
let firstRegExp = new RegExp(firstPattern);

When the expression is evaluated, the results are incorrect because the caret is a regular expression special character representing the beginning of the input. The beginning of the subject string will therefore be the only match:

let firstResult = firstRegExp.exec(inputStr);
console.log(firstResult); // [ '', index: 0, input: 'a^2' ]

The developer realizes he needs to escape the special caret character and therefore redefines his pattern with the standard backslash escape character:

let secondPattern = '\^';
let secondRegExp = new RegExp(secondPattern);
let secondResult = secondRegExp.exec(inputStr);
console.log(secondResult); // [ '', index: 0, input: 'a^2' ]

When the developer evaluates the second expression, however, the results are the same. This is because JavaScript will first evaluate the backslash escape character in the pattern string, before it ever reaches the RegExp constructor. When the developer eexamines the second pattern string, it is identical to the first despite the added backslash.

console.log(firstPattern === secondPattern); // true

Finally, the developer adds a second backslash to the pattern string, causing the JavaScript interpreter to preserve a single backslash character in the string stored in memory. This single backslash is successfully passed to the RegExp constructor, the expression therefore interprets the caret character literally, and the caret is correctly matched inside the input string:

let finalPattern = '\\^';
let finalRegExp = new RegExp(finalPattern);
let finalResult = finalRegExp.exec(inputStr);
console.log(finalResult); // [ '^', index: 1, input: 'a^2' ]

This is essentially a simple case of double-escaping. This module simply saves developers the trouble of determining the set of JavaScript's regular expression special characters and writing a function to escape them.

Development

To set up an optional development environment, a Vagrantfile is included. Install Vagrant and run:

vagrant up

Once Vagrant has completed provisioning, vagrant ssh into the box and run the tests with:

make

Feedback

I wrote this miniature module to practice with a few of the tools, libraries, and workflows available to JS developers. I welcome constructive criticism and advice.