Regex Lookahead

Summary: in this tutorial, you’ll learn to use the regex lookahead and negative lookahead.

Introduction to the regex lookahead #

Sometimes, you want to match A but only if it is followed by B. For example, suppose you have the following string:

2 chicken weigh 30lb

And you want to match the number (30) followed by the string lb, not the number 2. In this case, you can use the regex lookahead with the following syntax:

A(?=B)

The lookahead means to search for A but matches only if followed by B. For a number followed by the string lb, you can use the following pattern:

\d+(?=lb)

In this pattern:

\d+ match one or more digits
?= is the lookahead
lb match the text lb.

The following code uses the regex lookahead syntax to match a number followed by the text lb:

<?php

$pattern = '/\d+(?=lb)/';
$str = '2 chicken weigh 30lb';

if (preg_match($pattern, $str, $matches)) {
    print_r($matches); // 30

}

Try it

Output:

Array
(
    [0] => 30
)

The following regular expression also matches 30 followed immediately by lb:

'/\d+(?=lb)/'

For example:

<?php

$pattern = '/\d+(?=lb)/';
$str = '2 chicken weigh 30lb';

if (preg_match($pattern, $str, $matches)) {
    print_r($matches); // 30

}

Try it

Output:

Array
(
    [0] => 30
)

Multiple lookaheads #

The following illustrates the multiple lookaheads:

A(?=B)(?=C)

It works like this:

Find A
Test if B is immediately after A, skip if it’s not.
Test if C is also immediately after B; skip if it’s not.
If both tests pass, the A is a match; otherwise, search for the next match.

In short, the A(?=B)(?=C) pattern matches A followed by B and C simultaneously.

Negative Lookahead #

Suppose you want to match only the number 2 in the following text but not the number 30:

2 chicken weigh 30lb

To do that, you can use the negative lookahead syntax:

A(?!B)

The A(?!B) matches A only if followed by B. It’s the \d+ not followed by the string lb:

<?php

$pattern = '/\d+(?!lb)/';
$str = '2 chicken weigh 30lb';

if (preg_match($pattern, $str, $matches)) {
    print_r($matches); // 2

}

Try it

Output:

Array
(
    [0] => 2
)

Summary #

Use the regex lookahead A(?=B) that matches A only if followed by B.
Use the negative regex lookahead A(?!B) that matches A only if not followed by B.

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