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10.3.2 RegExp Methods



10.3.2 RegExp Methods

RegExp objects define two methods that perform pattern-matching operations; they behave similarly to the String methods described earlier. The main RegExp pattern-matching method is exec(). It is similar to the String match() method described in §10.2 , except that it is a RegExp method that takes a string, rather than a String method that takes a RegExp. The exec() method executes a regular expression on the specified string. That is, it searches the string for a match. If it finds none, it returns null. If it does find one, however, it returns an array just like the array returned by the match() method for nonglobal searches. Element 0 of the array contains the string that matched the regular expression, and any subsequent array elements contain the substrings that matched any parenthesized subexpressions. Furthermore, the index property contains the character position at which the match occurred, and the input property refers to the string that was searched.

Unlike the match() method, exec() returns the same kind of array whether or not the regular expression has the global g flag. Recall that match() returns an array of matches when passed a global regular expression. exec(), by contrast, always returns a single match and provides complete information about that match. When exec() is called on a regular expression that has the g flag, it sets the lastIndex property of the regular-expression object to the character position immediately following the matched substring. When exec() is invoked a second time for the same regular expression, it begins its search at the character position indicated by the lastIndex property. If exec() does not find a match, it resets lastIndex to 0. (You can also set lastIndex to 0 at any time, which you should do whenever you quit a search before you find the last match in one string and begin searching another string with the same RegExp object.) This special

262 | Chapter 10: Pattern Matching with Regular Expressions

behavior allows you to call exec() repeatedly in order to loop through all the regular expression matches in a string. For example:

var pattern = /Java/g;

var text = "JavaScript is more fun than Java!";

var result;

while((result = pattern.exec(text)) != null) {

alert("Matched '" + result[0] + "'" +

" at position " + result.index +

"; next search begins at " + pattern.lastIndex);

}

The other RegExp method is test(). test() is a much simpler method than exec(). It takes a string and returns true if the string contains a match for the regular expression:

var pattern = /java/i;

pattern.test("JavaScript"); // Returns true

Calling test() is equivalent to calling exec() and returning true if the return value of exec() is not null. Because of this equivalence, the test() method behaves the same way as the exec() method when invoked for a global regular expression: it begins searching the specified string at the position specified by lastIndex, and if it finds a match, it sets lastIndex to the position of the character immediately following the match. Thus, you can loop through a string using the test() method just as you can with the exec() method.

The String methods search(), replace(), and match()do not use the lastIndexproperty as exec() and test() do. In fact, the String methods simply reset lastIndex to 0. If you use exec() or test() on a pattern that has the g flag set, and you are searching multiple strings, you must either find all the matches in each string so that lastIndex is automatically reset to zero (this happens when the last search fails), or you must explicitly set the lastIndexproperty to 0 yourself. If you forget to do this, you may start searching a new string at some arbitrary position within the string rather than from the beginning. If your RegExp doesn’t have the g flag set, then you don’t have to worry about any of this, of course. Keep in mind also that in ECMAScript 5 each evaluation of a regular expression literal creates a new RegExp object with its own lastIndex property, and this reduces the risk of accidentally using a “leftover” lastIndex value.

10.3 The RegExp Object | 263

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