RegExp() constructor

The RegExp constructor creates a regular expression object for matching text with a pattern.

For an introduction to regular expressions, read the Regular Expressions chapter in the JavaScript Guide.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

Literal, constructor, and factory notations are possible:

/pattern/flags
new RegExp(pattern[, flags])
RegExp(pattern[, flags])

Parameters

pattern

The text of the regular expression.

As of ES5, this can also be another RegExp object or literal (for the two RegExp constructor notations only). Patterns may include special characters to match a wider range of values than would a literal string. 

flags

If specified, flags is a string that contains the flags to add.

Alternatively, if an object is supplied for the pattern, the flags string will replace any of that object's flags (and lastIndex will be reset to 0) (as of ES2015).

If flags is not specified and a regular expressions object is supplied, that object's flags (and lastIndex value) will be copied over.

flags may contain any combination of the following characters:

g (global match)

Find all matches rather than stopping after the first match.

i (ignore case)

If u flag is also enabled, use Unicode case folding.

m (multiline)

Treat beginning and end characters (^ and $) as working over multiple lines. In other words, match the beginning or end of each line (delimited by \n or \r), not only the very beginning or end of the whole input string.

s ("dotAll")

Allows . to match newlines.

u (unicode)

Treat pattern as a sequence of Unicode code points. (See also Binary strings).

y (sticky)

Matches only from the index indicated by the lastIndex property of this regular expression in the target string. Does not attempt to match from any later indexes.

Examples

Literal notation and constructor

There are two ways to create a RegExp object: a literal notation and a constructor.

• The literal notation's parameters are enclosed between slashes and do not use quotation marks.
• The constructor function's parameters are not enclosed between slashes but do use quotation marks.

The following three expressions create the same regular expression:

/ab+c/i
new RegExp(/ab+c/, 'i') // literal notation
new RegExp('ab+c', 'i') // constructor

The literal notation results in compilation of the regular expression when the expression is evaluated. Use literal notation when the regular expression will remain constant. For example, if you use literal notation to construct a regular expression used in a loop, the regular expression won't be recompiled on each iteration.

The constructor of the regular expression object—for example, new RegExp('ab+c')—results in runtime compilation of the regular expression. Use the constructor function when you know the regular expression pattern will be changing, or you don't know the pattern and are getting it from another source, such as user input.

Specifications

Browser compatibility

See also

• Regular Expressions chapter in the JavaScript Guide
• String.prototype.match()
• String.prototype.replace()