Regex Practice Lesson 9 of 14

Lookahead

Every pattern so far consumes what it matches - the matched text becomes part of the result. A lookahead, (?=...), is different: it checks that something comes next without consuming it. /cat(?= food)/ only matches "cat" when it's immediately followed by " food" - but the match itself is still just "cat", not "cat food".

Lookahead is also how you check "does this string contain X somewhere" without pinning down exactly where. /^(?=.*\d)/ reads as: from the start, look ahead for .*\d (anything, then a digit) - true if a digit exists anywhere in the string, even though the lookahead itself matches zero characters. This is the trick behind password rules like "must contain a digit" - no loop, no scanning code, just a position check.

Your task: write matchCatFood(str), returning "cat" if str contains "cat" immediately followed by " food" (without including " food" in the result), or null otherwise; and hasDigitSomewhere(str), true if str contains a digit anywhere.

You'll practice:

Asserting what comes next with (?=...) without consuming it
Using a lookahead to check "contains X somewhere" in one pattern

// Write matchCatFood(str): "cat" if immediately followed by " food"
// (without " food" in the result), or null.
function matchCatFood(str) {

}

// Write hasDigitSomewhere(str): true if str contains a digit anywhere.
function hasDigitSomewhere(str) {

}