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reluctant quantifier

Boundary

Description

Beginning of the line

End of the line

Word boundary

Non-word boundary

Suppose that you want to find a word, but only if it appears at the end of the line. You can use the dollar

sign (

) to indicate this:

var sToMatch = “Important word is the last one.”;

var reLastWord = /(\w+)\.$/;

reLastWord.test(sToMatch);

alert(RegExp.$1); //outputs “one”

The regular expression in this example looks for the last word with one or more word characters preced-

ing a period that appears before the end of the line. When this expression is run against

sToMatch

, it

returns

“one”

. You can easily change this expression to get the first word in the line by using the caret

(

) character:

var sToMatch = “Important word is the last one.”;

var reFirstWord = /^(\w+)/;

reFirstWord.test(sToMatch);

alert(RegExp.$1); //outputs “Important”

In this example, the regular expression looks for the beginning of the line followed by one or more word

characters. If a non-word character is encountered, the match stops, returning

“Important”

. This exam-

ple can be easily updated to use a word boundary instead:

var sToMatch = “Important word is the last one.”;

var reFirstWord = /^(.+?)\b/;

reFirstWord.test(sToMatch);

alert(RegExp.$1); //outputs “Important”

Here, the regular expression uses a reluctant quantifier to specify any character can appear one or more

times before a word boundary (if a greedy quantifier is used, the expression matches the entire string).

Using the word boundary is a great way to extract words from a string.

var sToMatch = “First second third fourth fifth sixth”

var reWords = /\b(\S+?)\b/g;

var arrWords = sToMatch.match(reWords);

The regular expression

reWords

uses both the word boundary (

) and the non-white space class (

)

to extract the words in a sentence. After execution, the

arrWords

array contains

“First”

“second”

“third”

“fourth”

“fifth”

, and

“sixth”

. Note that the beginning of the line and the end of the line,

normally represented by

and

, respectively, both count as word boundaries so

“First”

and

“sixth”

are included in the result. This is not the only way to get all the words in a sentence, however.

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Regular Expressions

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