RegExp.prototype.test()

The test() method executes a search for a match between a regular expression and a specified string. Returns true or false.

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Syntax

regexObj.test(str)

Parameters

str

The string against which to match the regular expression.

Returns

true if there is a match between the regular expression and the specified string; otherwise, false.

Description

Use test() whenever you want to know whether a pattern is found in a string. test() returns a boolean, unlike the String.prototype.search() method, which returns the index (or -1 if not found). To get more information (but with slower execution), use the exec() method (similar to the String.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()

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

var str = 'hello world!';
var 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) {
  var midstring;
  if (re.test(str)) {
    midstring = ' contains ';
  } else {
    midstring = ' does not contain ';
  }
  console.log(str + midstring + re.source);
}

Using test() on a regex with the global flag

If the regex has the global flag set, test() will advance the lastIndex of the regex. A subsequent use of test() will start the search at the substring of str specified by lastIndex (exec() will also advance the lastIndex property). It is worth noting that the lastIndex will not reset when testing a different string.

The following example demonstrates this behaviour:

var regex = /foo/g;

// 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 3
regex.test('foobar') //false

Using the same mechanism the following example counts the total number of words in a string:

function countWords (sText) {

  for (var rWord = /\w+/g, nCount = 0; rWord.test(sText); nCount++);

  return nCount;

}

console.log(countWords("What a beautiful day!")); // 4

Specifications

Browser compatibility

Firefox-specific notes

Prior to Firefox 8, test() was implemented incorrectly; when it was called with no parameters, it would match against the value of the previous input (RegExp.input property) instead of against the string "undefined". This is fixed; now /undefined/.test() correctly results in true, instead of an error.

See also

• Regular Expressions chapter in the JavaScript Guide
• RegExp