Javascript Regular Expression : Positive Look Ahead

 

JavaScript allows you to specify that a portion of a regular expression matches

only if it is or is not followed by a particular subexpression. The (?=) syntax

specifies a positive lookahead; it only matches the previous item if the item

is followed immediately by the expression contained in (?=). The lookahead

expression is not included in the match.

lets suppose you want to match all those USD characters which are

followed by some digits. Simply you want to match all those USD

characters which are immediately followed by numbers for example

you want to match

Code:

//positive look ahead

var line ="USD 100 USD 350 USD 345.56 USD currency USD rate"

var pattern=/USD(?=\s\d+\.{0,}\d+)/gm

var result = line.match(pattern)

console.log(result)

var lastIndex

while ((match = pattern.exec(line))) {

lastIndex = match.index;

console.log("Occurance At:" + lastIndex)

}

Output:

[ 'USD', 'USD', 'USD' ]

Occurance At:0

Occurance At:8

Occurance At:16

Explanation:

Here first the engine will search for U after finding U upper case the

engine will look for S if it finds it will see if it is immediately

followed by D.

In case of a USD match the engine will enter lookahead and finds that it

is a positive lookahead and in this look ahead there is a space followed

by an optional number one or more quantifier then an optional dot and

after dot there is one or more digits.

This regex will match all USD words followed by a number of one or more

digits.