RegExp.prototype.test() - JavaScript | MDN (original) (raw)

Baseline

Widely available

The test() method of RegExp instances executes a search with this regular expression for a match between a regular expression and a specified string. Returns true if there is a match; false otherwise.

JavaScript RegExp objects are stateful when they have the global or sticky flags set (e.g., /foo/g or /foo/y). They store alastIndex from the previous match. Using this internally, test() can be used to iterate over multiple matches in a string of text (with capture groups).

Try it

const str = "table football";

const regex = new RegExp("foo*");
const globalRegex = new RegExp("foo*", "g");

console.log(regex.test(str));
// Expected output: true

console.log(globalRegex.lastIndex);
// Expected output: 0

console.log(globalRegex.test(str));
// Expected output: true

console.log(globalRegex.lastIndex);
// Expected output: 9

console.log(globalRegex.test(str));
// Expected output: false

Syntax

Parameters

str

The string against which to match the regular expression. All values are coerced to strings, so omitting it or passing undefined causes test() to search for the string "undefined", which is rarely what you want.

Return value

true if there is a match between the regular expression and the stringstr. Otherwise, false.

Description

Use test() whenever you want to know whether a pattern is found in a string. test() returns a boolean, unlike theString.prototype.search() method (which returns the index of a match, or-1 if not found).

To get more information (but with slower execution), use theexec() method. (This is similar to theString.prototype.match() method.)

As with exec() (or in combination with it), test() called multiple times on the same global regular expression instance will advance past the previous match.

Examples

Using test()

This example tests if "hello" is contained at the very beginning of a string, returning a boolean result.

const str = "hello world!";
const result = /^hello/.test(str);

console.log(result); // true

The following example logs a message which depends on the success of the test:

function testInput(re, str) {
  const midString = re.test(str) ? "contains" : "does not contain";
  console.log(`${str} <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>m</mi><mi>i</mi><mi>d</mi><mi>S</mi><mi>t</mi><mi>r</mi><mi>i</mi><mi>n</mi><mi>g</mi></mrow><annotation encoding="application/x-tex">{midString} </annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.8889em;vertical-align:-0.1944em;"></span><span class="mord"><span class="mord mathnormal">mi</span><span class="mord mathnormal">d</span><span class="mord mathnormal">St</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal">in</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span></span></span></span></span>{re.source}`);
}

Using test() on a regex with the "global" flag

When a regex has the global flag set,test() will advance the lastIndex of the regex. (RegExp.prototype.exec() also advances the lastIndex property.)

Further calls to test(str) will resume searchingstr starting from lastIndex. ThelastIndex property will continue to increase each time test()returns true.

**Note:**As long as test() returns true,lastIndex will not reset—even when testing a different string!

When test() returns false, the calling regex'slastIndex property will reset to 0.

The following example demonstrates this behavior:

const regex = /foo/g; // the "global" flag is set

// regex.lastIndex is at 0
regex.test("foo"); // true

// regex.lastIndex is now at 3
regex.test("foo"); // false

// regex.lastIndex is at 0
regex.test("barfoo"); // true

// regex.lastIndex is at 6
regex.test("foobar"); // false

// regex.lastIndex is at 0
regex.test("foobarfoo"); // true

// regex.lastIndex is at 3
regex.test("foobarfoo"); // true

// regex.lastIndex is at 9
regex.test("foobarfoo"); // false

// regex.lastIndex is at 0
// (...and so on)

Specifications

Browser compatibility

See also