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Lookaheads

Sometimes you may want to capture a particular group of characters only if they appear before another

set of characters. Using lookaheads makes this process easy.

lookahead

is just what it sounds like: It tells the regular expression evaluator to look ahead any number

of characters without losing its spot. There are both positive and negative lookaheads. Positive lookaheads

check whether a certain set of characters comes next. Negative lookaheads determine if a certain set of

characters does not come next.

A positive lookahead is created by enclosing a pattern between

(?=

and

)

. Note that this is not a group,

even though it uses parentheses. In fact, groups don’t recognize that lookaheads (either positive or nega-

tive) exist. Consider the following:

var sToMatch1 = “bedroom”;

var sToMatch2 = “bedding”;

var reBed = /(bed(?=room))/;

alert(reBed.test(sToMatch1)); //outputs “true”

alert(RegExp.$1); //outputs “bed”

alert(reBed.test(sToMatch2)); //outputs “false”

In this example,

reBed

matches

“bed”

only if it is followed by

“room”

. Therefore, it matches

sToMatch1

but not

sToMatch2

. After testing the expression against

sToMatch1

, this code outputs the

contents of

RegExp.$1

, which is

“bed”

, not

“bedroom”

. The

“room”

part of the pattern is contained

inside of a lookahead and so isn’t returned as part of the group.

At the other end of the spectrum is a negative lookahead, created by enclosing a pattern between

(?!

and

)

. The previous example can be changed to use a negative lookahead to match

“bedding”

instead

“bedroom”

var sToMatch1 = “bedroom”;

var sToMatch2 = “bedding”;

var reBed = /(bed(?!room))/;

alert(reBed.test(sToMatch1)); //outputs “false”

alert(reBed.test(sToMatch2)); //outputs “true”

alert(RegExp.$1); //outputs “bed”

Here, the expression is changed to match

“bed”

only if

“room”

doesn’t follow it, so the pattern matches

“bedding”

but not

“bedroom”

. After testing against

sToMatch2

RegExp.$1

contains

“bed”

once

again, not

“bedding”

Boundaries

are used in regular expressions to indicate the location of a pattern. The following table lists

the available boundaries:

Although JavaScript supports regular-expression lookaheads, it does not support

lookbehinds, which match patterns such as

“match b only if it isn’t pre-

ceded by a”

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