RegExp() constructor
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The RegExp()
constructor creates RegExp
objects.
For an introduction to regular expressions, read the Regular Expressions chapter in the JavaScript Guide.
Try it
Syntax
new RegExp(pattern)
new RegExp(pattern, flags)
RegExp(pattern)
RegExp(pattern, flags)
Note: RegExp()
can be called with or without new
, but sometimes with different effects. See Return value.
Parameters
pattern
-
The text of the regular expression. This can also be another
RegExp
object. flags
Optional-
If specified,
flags
is a string that contains the flags to add. Alternatively, if aRegExp
object is supplied for thepattern
, theflags
string will replace any of that object's flags (andlastIndex
will be reset to0
).flags
may contain any combination of the following characters:d
(indices)-
Generate indices for substring matches.
g
(global)-
Find all matches rather than stopping after the first match.
i
(ignore case)-
When matching, casing differences are ignored.
m
(multiline)-
Treat beginning and end assertions (
^
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. v
(unicodeSets)-
An upgrade to the
u
flag that enables set notation in character classes as well as properties of 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.
Return value
RegExp(pattern)
returns pattern
directly if all of the following are true:
RegExp()
is called withoutnew
;pattern
is a regex;pattern.constructor === RegExp
(usually meaning it's not a subclass);flags
isundefined
.
In all other cases, calling RegExp()
with or without new
both create a new RegExp
object. If pattern
is a regex, the new object's source is pattern.source
; otherwise, its source is pattern
coerced to a string. If the flags
parameter is not undefined
, the new object's flags
is the parameter's value; otherwise, its flags
is pattern.flags
(if pattern
is a regex).
Exceptions
SyntaxError
-
Thrown in one of the following cases:
pattern
cannot be parsed as a valid regular expression.flags
contains repeated characters or any character outside of those allowed.
Examples
Literal notation and constructor
There are two ways to create a RegExp
object: a literal notation and a constructor.
- The literal notation takes a pattern between two slashes, followed by optional flags, after the second slash.
- The constructor function takes either a string or a
RegExp
object as its first parameter and a string of optional flags as its second parameter.
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
Before regular expressions can be used, they have to be compiled. This process allows them to perform matches more efficiently. There are two ways to compile and get a RegExp
object.
The literal notation results in compilation of the regular expression when the expression is evaluated. On the other hand, the constructor of the RegExp
object, new RegExp('ab+c')
, results in runtime compilation of the regular expression.
Use a string as the first argument to the RegExp()
constructor when you want to build the regular expression from dynamic input.
Building a regular expression from dynamic inputs
const breakfasts = ["bacon", "eggs", "oatmeal", "toast", "cereal"];
const order = "Let me get some bacon and eggs, please";
order.match(new RegExp(`\\b(${breakfasts.join("|")})\\b`, "g"));
// Returns ['bacon', 'eggs']
Specifications
Specification |
---|
ECMAScript Language Specification # sec-regexp-constructor |
Browser compatibility
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