Why does Javascript’s regex.exec() not always return the same value? [duplicate]

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A JavaScript RegExp object is stateful.

When the regex is global, if you call a method on the same regex object, it will start from the index past the end of the last match.

When no more matches are found, the index is reset to 0 automatically.

To reset it manually, set the lastIndex property.

reg.lastIndex = 0;

This can be a very useful feature. You can start the evaluation at any point in the string if desired, or if in a loop, you can stop it after a desired number of matches.

Here’s a demonstration of a typical approach to using the regex in a loop. It takes advantage of the fact that exec returns null when there are no more matches by performing the assignment as the loop condition.

var re = /foo_(\d+)/g,
    str = "text foo_123 more text foo_456 foo_789 end text",
    match,
    results = [];

while (match = re.exec(str))
    results.push(+match[1]);

DEMO: http://jsfiddle.net/pPW8Y/

If you don’t like the placement of the assignment, the loop can be reworked, like this for example…

var re = /foo_(\d+)/g,
    str = "text foo_123 more text foo_456 foo_789 end text",
    match,
    results = [];

do {
    match = re.exec(str);
    if (match)
        results.push(+match[1]);
} while (match);

DEMO: http://jsfiddle.net/pPW8Y/1/

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