[Contents] [Previous] [Next] [Index]

Chapter 9
Expressions and Operators

This chapter describes JavaScript expressions and operators, including assignment, comparison, arithmetic, bitwise, logical, string, and special operators. It also describes regular expressions.

Expressions

An expression is any valid set of literals, variables, operators, and expressions that evaluates to a single value; the value can be a number, a string, or a logical value.

Conceptually, there are two types of expressions: those that assign a value to a variable, and those that simply have a value. For example, the expression x = 7 is an expression that assigns x the value seven. This expression itself evaluates to seven. Such expressions use assignment operators. On the other hand, the expression 3 + 4 simply evaluates to seven; it does not perform an assignment. The operators used in such expressions are referred to simply as operators.

JavaScript has the following types of expressions:

The special keyword null denotes a null value. In contrast, variables that have not been assigned a value are undefined and cause a runtime error if used as numbers or as numeric variables. Array elements that have not been assigned a value, however, evaluate to false. For example, the following code executes the function myFunction because the array element is not defined:

myArray=new Array()
if (!myArray["notThere"])
   myFunction()

Operators

JavaScript has assignment, comparison, arithmetic, bitwise, logical, string, and special operators. This section describes the operators and contains information about operator precedence.

JavaScript has both binary and unary operators. A binary operator requires two operands, one before the operator and one after the operator:

operand1 operator operand2
For example, 3+4 or x*y.

A unary operator requires a single operand, either before or after the operator:

operator operand
or

operand operator
For example, x++ or ++x.

In addition, JavaScript has one ternary operator, the conditional operator. A ternary operator requires three operands.

Assignment Operators

An assignment operator assigns a value to its left operand based on the value of its right operand. The basic assignment operator is equal (=), which assigns the value of its right operand to its left operand. That is, x = y assigns the value of y to x.

The other assignment operators are shorthand for standard operations, as shown in Table 9.1.

Table 9.1 Assignment operators
Shorthand operator Meaning
x += y
x = x + y
x -= y 
x = x - y
x *= y 
x = x * y
x /= y 
x = x / y
x %= y 
x = x % y
x <<= y 
x = x << y
x >>= y 
x = x >> y
x >>>= y
x = x >>> y
x &= y
x = x & y
x ^= y
x = x ^ y
x |= y
x = x | y

Comparison Operators

A comparison operator compares its operands and returns a logical value based on whether the comparison is true or not. The operands can be numerical or string values. When used on string values, the comparisons are based on the standard lexicographical ordering. They are described in Table 9.2.

Table 9.2 Comparison operators
Operator Description Examples returning true1
Equal (==)

Returns true if the operands are equal.

3 == var1
Not equal (!=)

Returns true if the operands are not equal.

var1 != 4
Greater than (>)

Returns true if left operand is greater than right operand.

var2 > var1
Greater than or equal (>=)

Returns true if left operand is greater than or equal to right operand.

var2 >= var1
var1 >= 3
Less than (<)

Returns true if left operand is less than right operand.

var1 < var2
Less than or equal (<=)

Returns true if left operand is less than or equal to right operand.

var1 <= var2
var2 <= 5
1 In these examples, assume var1 has been assigned the value 3 and var2 had been assigned the value 4.

Arithmetic Operators

Arithmetic operators take numerical values (either literals or variables) as their operands and return a single numerical value. The standard arithmetic operators are addition (+), subtraction (-), multiplication (*), and division (/). These operators work as they do in other programming languages.

In addition, JavaScript provides the arithmetic operators listed in Table 9.3.

Table 9.3 Arithmetic Operators
Operator Description Example
%
(Modulus)

Binary operator. Returns the integer remainder of dividing the 2 operands.

12 % 5 returns 2.

++
(Increment)

Unary operator. Adds one to its operand. If used as a prefix operator (++x), returns the value of its operand after adding one; if used as a postfix operator (x++), returns the value of its operand before adding one.

If x is 3, then ++x sets x to 4 and returns 4, whereas x++ sets x to 4 and returns 3.

--
(Decrement)

Unary operator. Subtracts one to its operand. The return value is analogous to that for the increment operator.

If x is 3, then --x sets x to 2 and returns 2, whereas x++ sets x to 2 and returns 3.

-
(Unary negation)

Unary operator. Returns the negation of its operand.

If x is 3, then -x returns -3.

Bitwise Operators

Bitwise operators treat their operands as a set of bits (zeros and ones), rather than as decimal, hexadecimal, or octal numbers. For example, the decimal number nine has a binary representation of 1001. Bitwise operators perform their operations on such binary representations, but they return standard JavaScript numerical values.

The following table summarizes JavaScript's bitwise operators

Table 9.4 Bitwise operators
Operator Usage Description
Bitwise AND

a & b
Returns a one in each bit position if bits of both operands are ones.

Bitwise OR

a | b
Returns a one in a bit if bits of either operand is one.

Bitwise XOR

a ^ b
Returns a one in a bit position if bits of one but not both operands are one.

Bitwise NOT

~ a
Flips the bits of its operand.

Left shift

a << b
Shifts a in binary representation b bits to left, shifting in zeros from the right.

Sign-propagating right shift

a >> b
Shifts a in binary representation b bits to right, discarding bits shifted off.

Zero-fill right shift

a >>> b
Shifts a in binary representation b bits to the right, discarding bits shifted off, and shifting in zeros from the left.

Bitwise Logical Operators

Conceptually, the bitwise logical operators work as follows:

For example, the binary representation of nine is 1001, and the binary representation of fifteen is 1111. So, when the bitwise operators are applied to these values, the results are as follows:

Bitwise Shift Operators

The bitwise shift operators take two operands: the first is a quantity to be shifted, and the second specifies the number of bit positions by which the first operand is to be shifted. The direction of the shift operation is controlled by the operator used.

Shift operators convert their operands to thirty-two-bit integers and return a result of the same type as the left operator.

The shift operators are listed in Table 9.5.

Table 9.5 Bitwise shift operators
Operator Description Example
<<
(Left shift)

This operator shifts the first operand the specified number of bits to the left. Excess bits shifted off to the left are discarded. Zero bits are shifted in from the right.

9<<2 yields 36, because 1001 shifted 2 bits to the left becomes 100100, which is 36.

>>
(Sign-propagating right shift)

This operator shifts the first operand the specified number of bits to the right. Excess bits shifted off to the right are discarded. Copies of the leftmost bit are shifted in from the left.

9>>2 yields 2, because 1001 shifted 2 bits to the right becomes 10, which is 2. Likewise, -9>>2 yields -3, because the sign is preserved.

>>>
(Zero-fill right shift)

This operator shifts the first operand the specified number of bits to the right. Excess bits shifted off to the right are discarded. Zero bits are shifted in from the left.

19>>>2 yields 4, because 10011 shifted 2 bits to the right becomes 100, which is 4. For non-negative numbers, zero-fill right shift and sign-propagating right shift yield the same result.

Logical Operators

Logical operators take Boolean (logical) values as operands and return a Boolean value. They are described in Table 9.6.

Table 9.6 Logical operators
Operator Usage Description Example1
and (&&)

expr1 && expr2
Returns expr1 if it converts to false. Otherwise, returns expr2.

var1 && var2 returns "Dog".
var2 && var3 returns false

or (||)

expr1 || expr2
Returns expr1 if it converts to true. Otherwise, returns expr2.

var1 || var2 returns "Cat".
var3 || var1 returns "Cat".
var3 || (3==4) returns false.

not (!)

!expr
If expr is true, returns false; if expr is false, returns true.

!var1 returns false.
!var3 returns true.

1 Assume var1 is "Cat", var2 is "Dog", and var3 is false.

Short-Circuit Evaluation

As logical expressions are evaluated left to right, they are tested for possible "short-circuit" evaluation using the following rules:

The rules of logic guarantee that these evaluations are always correct. Note that the anything part of the above expressions is not evaluated, so any side effects of doing so do not take effect.

String Operators

In addition to the comparison operators, which can be used on string values, the concatenation operator (+) concatenates two string values together, returning another string that is the union of the two operand strings. For example, "my " + "string" returns the string "my string".

The shorthand assignment operator += can also be used to concatenate strings. For example, if the variable mystring has the value "alpha," then the expression mystring += "bet" evaluates to "alphabet" and assigns this value to mystring.

Special Operators

conditional operator

The conditional operator is the only JavaScript operator that takes 3 operands. The operator can have one of two values based on a condition. The syntax is

(condition) ? val1 : val2
If condition is true, the operator has the value of val1. Otherwise it has the value of val2. You can use the conditional operator anywhere you would use a standard operator.

For example,

status = (age >= 18) ? "adult" : "minor"
This statement assigns the value "adult" to the variable status if age is eighteen or more. Otherwise, it assigns the value "minor" to status.

comma operator

The comma operator (,) simply evaluates both of its operands and returns the value of the second operand. This operator is primarily used inside a for loop, to allow multiple variables to be updated each time through the loop.

For example, if a is a 2-dimensional array with 10 elements on a side, the following code uses the comma operator to increment two variables at once. The code prints the values of the diagonal elements in the array:

for (var i=0, j=10; i <= 10; i++, j--)
   document.writeln("a["+i+","+j+"]= " + a[i,j])

delete

The delete operator deletes an object's property or an element at a specified index in an array. Its syntax is:

delete objectName.property
delete objectname[index]
delete property
where objectName is the name of an object, property is an existing property, and index is an integer representing the location of an element in an array

The third form is legal only within a with statement. If the deletion succeeds, the delete operator sets the property or element to undefined. delete always returns undefined.

new

You can use the new operator to create an instance of a user-defined object type or of one of the predefined object types Array, Boolean, Date, Function, Image, Number, Object, Option, RegExp, or String. On the server, you can also use it with DbPool, Lock, File, or SendMail. Use new as follows:

objectName = new objectType ( param1 [,param2] ...[,paramN] )
For more information, see new in the JavaScript Reference.

typeof

The typeof operator is used in either of the following ways:

1. typeof operand
2. typeof (operand)
The typeof operator returns a string indicating the type of the unevaluated operand. operand is the string, variable, keyword, or object for which the type is to be returned. The parentheses are optional.

Suppose you define the following variables:

var myFun = new Function("5+2")
var shape="round"
var size=1
var today=new Date()
The typeof operator returns the following results for these variables:

typeof myFun is object
typeof shape is string
typeof size is number
typeof today is object
typeof dontExist is undefined
For the keywords true and null, the typeof operator returns the following results:

typeof true is boolean
typeof null is object
For a number or string, the typeof operator returns the following results:

typeof 62 is number
typeof 'Hello world' is string
For property values, the typeof operator returns the type of value the property contains:

typeof document.lastModified is string
typeof window.length is number
typeof Math.LN2 is number
For methods and functions, the typeof operator returns results as follows:

typeof blur is function
typeof eval is function
typeof parseInt is function
typeof shape.split is function
For predefined objects, the typeof operator returns results as follows:

typeof Date is function
typeof Function is function
typeof Math is function
typeof Option is function
typeof String is function

void

The void operator is used in either of the following ways:

1. javascript:void (expression)
2. javascript:void expression
The void operator specifies an expression to be evaluated without returning a value. expression is a JavaScript expression to evaluate. The parentheses surrounding the expression are optional, but it is good style to use them.

You can use the void operator to specify an expression as a hypertext link. The expression is evaluated but is not loaded in place of the current document.

The following code creates a hypertext link that does nothing when the user clicks it. When the user clicks the link, void(0) evaluates to 0, but that has no effect in JavaScript.

<A HREF="javascript:void(0)">Click here to do nothing</A>
The following code creates a hypertext link that submits a form when the user clicks it.

<A HREF="javascript:void(document.form.submit())">
Click here to submit</A>

Operator Precedence

The precedence of operators determines the order they are applied when evaluating an expression. You can override operator precedence by using parentheses.

Table 9.7 describes the precedence of operators, from lowest to highest.

Table 9.7 Operator precedence  
Operator type Individual operators
assignment

 = += -= *= /= %= <<= >>= >>>= &= ^= |=
conditional

?:
logical-or

 ||
logical-and

 &&
bitwise-or

 |
bitwise-xor

 ^
bitwise-and

 &
equality

 == !=
relational

 < <= > >=
bitwise shift

 << >> >>>
addition/subtraction

 + -
multiply/divide

 * / %
negation/increment

! ~ - ++ -- typeof void
call, member

 () [] .

Regular Expressions

JavaScript 1.2, available in Navigator 4.0, adds regular expressions to the language. Regular expressions are patterns used to match character combinations in strings. In JavaScript, regular expressions are also objects. For example, to search for all occurrences of 'the' in a string, you create a pattern consisting of 'the' and use the pattern to search for its match in a string. Regular expression patterns can be constructed using either object initializers (for example, /abc/) or the RegExp constructor function (for example, re = new RegExp("abc")). Object initializers are discussed in "Using Object Initializers".

These patterns are used with the exec and test methods of regular expressions, and with the match, replace, search, and split methods of String.

Creating a Regular Expression

You construct a regular expression in one of two ways:

Writing a Regular Expression Pattern

A regular expression pattern is composed of simple characters, such as /abc/, or a combination of simple and special characters, such as /ab*c/ or /Chapter (\d+)\.\d*/. The last example includes parentheses which are used as a memory device. The match made with this part of the pattern is remembered for later use, as described in "Using Parenthesized Substring Matches".

Using Simple Patterns

Simple patterns are constructed of characters for which you want to find a direct match. For example, the pattern /abc/ matches character combinations in strings only when exactly the characters 'abc' occur together and in that order. Such a match would succeed in the strings "Hi, do you know your abc's?" and "The latest airplane designs evolved from slabcraft." In both cases the match is with the substring 'abc'. There is no match in the string "Grab crab" because it does not contain the substring 'abc'.

Using Special Characters

When the search for a match requires something more than a direct match, such as finding one or more b's, or finding whitespace, the pattern includes special characters. For example, the pattern /ab*c/ matches any character combination in which a single 'a' is followed by zero or more 'b's (* means 0 or more occurrences of the preceding character) and then immediately followed by 'c'. In the string "cbbabbbbcdebc," the pattern matches the substring 'abbbbc'.

Table 9.8 provides a complete list and description of the special characters that can be used in regular expressions.

Table 9.8 Special characters in regular expressions.  
Character Meaning
\
For characters that are usually treated literally, indicates that the next character is special and not to be interpreted literally.

For example, /b/ matches the character 'b'. By placing a backslash in front of b, that is by using /\b/, the character becomes special to mean match a word boundary.

-or-

For characters that are usually treated specially, indicates that the next character is not special and should be interpreted literally.

For example, * is a special character that means 0 or more occurrences of the preceding character should be matched; for example, /a*/ means match 0 or more a's. To match * literally, precede the it with a backslash; for example, /a\*/ matches 'a*'.

^
Matches beginning of input or line.

For example, /^A/ does not match the 'A' in "an A," but does match it in "An A."

$
Matches end of input or line.

For example, /t$/ does not match the 't' in "eater", but does match it in "eat"

*
Matches the preceding character 0 or more times.

For example, /bo*/ matches 'boooo' in "A ghost booooed" and 'b' in "A bird warbled", but nothing in "A goat bleated".

+
Matches the preceding character 1 or more times. Equivalent to {1,}.

For example, /a+/ matches the 'a' in "candy" and all the a's in "caaaaaaandy."

?
Matches the preceding character 0 or 1 time.

For example, /e?le?/ matches the 'el' in "angel" and the 'le' in "angle."

.
(The decimal point) matches any single character except the newline character.

For example, /.n/ matches 'an' and 'on' in "nay, an apple is on the tree", but not 'nay'.

(x)
Matches 'x' and remembers the match.

For example, /(foo)/ matches and remembers 'foo' in "foo bar." The matched substring can be recalled from the resulting array's elements [1], ..., [n], or from the predefined RegExp object's properties $1, ..., $9.

x|y
Matches either 'x' or 'y'.

For example, /green|red/ matches 'green' in "green apple" and 'red' in "red apple."

{n}
Where n is a positive integer. Matches exactly n occurrences of the preceding character.

For example, /a{2}/ doesn't match the 'a' in "candy," but it matches all of the a's in "caandy," and the first two a's in "caaandy."

{n,}
Where n is a positive integer. Matches at least n occurrences of the preceding character.

For example, /a{2,} doesn't match the 'a' in "candy", but matches all of the a's in "caandy" and in "caaaaaaandy."

{n,m}
Where n and n are positive integers. Matches at least n and at most n occurrences of the preceding character.

For example, /a{1,3}/ matches nothing in "cndy", the 'a' in "candy," the first two a's in "caandy," and the first three a's in "caaaaaaandy" Notice that when matching "caaaaaaandy", the match is "aaa", even though the original string had more a's in it.

[xyz]
A character set. Matches any one of the enclosed characters. You can specify a range of characters by using a hyphen.

For example, [abcd] is the same as [a-c]. They match the 'b' in "brisket" and the 'c' in "ache".

[^xyz]
A negated or complemented character set. That is, it matches anything that is not enclosed in the brackets. You can specify a range of characters by using a hyphen.

For example, [^abc] is the same as [^a-c]. They initially match 'r' in "brisket" and 'h' in "chop."

[\b]
Matches a backspace. (Not to be confused with \b.)

\b
Matches a word boundary, such as a space. (Not to be confused with [\b].)

For example, /\bn\w/ matches the 'no' in "noonday";/\wy\b/ matches the 'ly' in "possibly yesterday."

\B
Matches a non-word boundary.

For example, /\w\Bn/ matches 'on' in "noonday", and /y\B\w/ matches 'ye' in "possibly yesterday."

\cX
Where X is a control character. Matches a control character in a string.

For example, /\cM/ matches control-M in a string.

\d
Matches a digit character. Equivalent to [0-9].

For example, /\d/ or /[0-9]/ matches '2' in "B2 is the suite number."

\D
Matches any non-digit character. Equivalent to [^0-9].

For example, /\D/ or /[^0-9]/ matches 'B' in "B2 is the suite number."

\f
Matches a form-feed.

\n
Matches a linefeed.

\r
Matches a carriage return.

\s
Matches a single white space character, including space, tab, form feed, line feed. Equivalent to [ \f\n\r\t\v].

for example, /\s\w*/ matches ' bar' in "foo bar."

\S
Matches a single character other than white space. Equivalent to [^ \f\n\r\t\v].

For example, /\S/\w* matches 'foo' in "foo bar."

\t
Matches a tab

\v
Matches a vertical tab.

\w
Matches any alphanumeric character including the underscore. Equivalent to [A-Za-z0-9_].

For example, /\w/ matches 'a' in "apple," '5' in "$5.28," and '3' in "3D."

\W 
Matches any non-word character. Equivalent to [^A-Za-z0-9_].

For example, /\W/ or /[^$A-Za-z0-9_]/ matches '%' in "50%."

\n
Where n is a positive integer. A back reference to the last substring matching the n parenthetical in the regular expression (counting left parentheses).

For example, /apple(,)\sorange\1/ matches 'apple, orange', in "apple, orange, cherry, peach." A more complete example follows this table.

Note: If the number of left parentheses is less than the number specified in \n, the \n is taken as an octal escape as described in the next row.

\ooctal
\xhex
Where \ooctal is an octal escape value or \xhex is a hexadecimal escape value. Allows you to embed ASCII codes into regular expressions.

Using Parentheses

Parentheses around any part of the regular expression pattern cause that part of the matched substring to be remembered. Once remembered, the substring can be recalled for other use, as described in "Using Parenthesized Substring Matches".

For example, the pattern /Chapter (\d+)\.\d*/ illustrates additional escaped and special characters and indicates that part of the pattern should be remembered. It matches precisely the characters 'Chapter ' followed by one or more numeric characters (\d means any numeric character and + means 1 or more times), followed by a decimal point (which in itself is a special character; preceding the decimal point with \ means the pattern must look for the literal character '.'), followed by any numeric character 0 or more times (\d means numeric character, * means 0 or more times). In addition, parentheses are used to remember the first matched numeric characters.

This pattern is found in "Open Chapter 4.3, paragraph 6" and '4' is remembered. The pattern is not found in "Chapters 3 and 4", because that string does not have a period after the '3'.

Working With Regular Expressions

Regular expressions are used with the regular expression methods test and exec and with the String methods match, replace, search, and split. These methods are explained in detail in the JavaScript Reference.

exec
A regular expression method that executes a search for a match in a string. It returns an array of information.

test
A regular expression method that tests for a match in a string. It returns true or false.

match
A String method that executes a search for a match in a string. It returns an array of information or null on a mismatch.

search
A String method that tests for a match in a string. It returns the index of the match, or -1 if the search fails.

replace
A String method that executes a search for a match in a string, and replaces the matched substring with a replacement substring.

split
A String method that uses a regular expression or a fixed string to break a string into an array of substrings.

When you want to know whether a pattern is found in a string, use the test or search method; for more information (but slower execution) use the exec or match methods. If you use exec or match and if the match succeeds, these methods return an array and update properties of the associated regular expression object and also of the predefined regular expression object, RegExp. If the match fails, the exec method returns null (which converts to false).

In the following example, the script uses the exec method to find a match in a string.

<SCRIPT LANGUAGE="JavaScript1.2">
myRe=/d(b+)d/g;
myArray = myRe.exec("cdbbdbsbz");
</SCRIPT>
If you do not need to access the properties of the regular expression, an alternative way of creating myArray is with this script:

<SCRIPT LANGUAGE="JavaScript1.2">
myArray = /d(b+)d/g.exec("cdbbdbsbz");
</SCRIPT>
If you want to be able to recompile the regular expression, yet another alternative is this script:

<SCRIPT LANGUAGE="JavaScript1.2">
myRe= new RegExp ("d(b+)d", "g:);
myArray = myRe.exec("cdbbdbsbz");
</SCRIPT>
With these scripts, the match succeeds and returns the array and updates the properties shown in Table 9.9.

Table 9.9 Results of regular expression execution.  
Object Property or Index Description In this example
myArray
The matched string and all remembered substrings

["dbbd", "bb"]

index
The 0-based index of the match in the input string

1

input
The original string

"cdbbdbsbz"

[0]
The last matched characters

"dbbd"

myRe
lastIndex
The index at which to start the next match. (This property is set only if the regular expression uses the g option, described in "Executing a Global Search and Ignoring Case".)

5

source
The text of the pattern

"d(b+)d"

RegExp
lastMatch
The last matched characters

"dbbd"

leftContext
The substring preceding the most recent match

"c"

rightContext
The substring following the most recent match

"bsbz"

RegExp.leftContext and RegExp.rightContext can be computed from the other values. RegExp.leftContext is equivalent to:

myArray.input.substring(0, myArray.index) 
and RegExp.rightContext is equivalent to:

myArray.input.substring(myArray.index + myArray[0].length)
As shown in the second form of this example, you can use the a regular expression created with an object initializer without assigning it to a variable. If you do, however, every occurrence is a new regular expression. For this reason, if you use this form without assigning it to a variable, you cannot subsequently access the properties of that regular expression. For example, assume you have this script:

<SCRIPT LANGUAGE="JavaScript1.2">
myRe=/d(b+)d/g;
myArray = myRe.exec("cdbbdbsbz");
document.writeln("The value of lastIndex is " + myRe.lastIndex);
</SCRIPT>
This script displays:

The value of lastIndex is 5
However, if you have this script:

<SCRIPT LANGUAGE="JavaScript1.2">
myArray = /d(b+)d/g.exec("cdbbdbsbz");
document.writeln("The value of lastIndex is " + /d(b+)d/g.lastIndex);
</SCRIPT>
It displays:

The value of lastIndex is 0
The occurrences of /d(b+)d/g in the two statements are different regular expression objects and hence have different values for their lastIndex property. If you need to access the properties of a regular expression created with an object initalizer, you should first assign it to a variable.

Using Parenthesized Substring Matches

Including parentheses in a regular expression pattern causes the corresponding submatch to be remembered. For example, /a(b)c/ matches the characters 'abc' and remembers 'b'. To recall these parenthesized substring matches, use the RegExp properties $1, ..., $9 or the Array elements [1], ..., [n].

The number of possible parenthesized substrings is unlimited. The predefined RegExp object holds up to the last nine and the returned array holds all that were found. The following examples illustrate how to use parenthesized substring matches.

Example 1. : The following script uses the replace method to switch the words in the string. For the replacement text, the script uses the values of the $1 and $2 properties.

<SCRIPT LANGUAGE="JavaScript1.2">
re = /(\w+)\s(\w+)/;
str = "John Smith";
newstr = str.replace(re, "$2, $1");
document.write(newstr)
</SCRIPT>
This prints "Smith, John".

Example 2. : In the following example, RegExp.input is set by the Change event. In the getInfo function, the exec method uses the value of RegExp.input as its argument. Note that RegExp must be prepended to its $ properties (because they appear outside the replacement string). (Example 3 is a more efficient, though possibly more cryptic, way to accomplish the same thing.)

<HTML>
<SCRIPT LANGUAGE="JavaScript1.2">
function getInfo(){
   re = /(\w+)\s(\d+)/
   re.exec();
   window.alert(RegExp.$1 + ", your age is " + RegExp.$2);
}
</SCRIPT>
Enter your first name and your age, and then press Enter.
<FORM>
<INPUT TYPE="text" NAME="NameAge" onChange="getInfo(this);">
</FORM>
</HTML>
Example 3. : The following example is similar to Example 2. Instead of using the RegExp.$1 and RegExp.$2, this example creates an array and uses a[1] and a[2]. It also uses the shortcut notation for using the exec method.

<HTML>
<SCRIPT LANGUAGE="JavaScript1.2">
function getInfo(){
   a = /(\w+)\s(\d+)/();
   window.alert(a[1] + ", your age is " + a[2]);
}
</SCRIPT>
Enter your first name and your age, and then press Enter.
<FORM>
<INPUT TYPE="text" NAME="NameAge" onChange="getInfo(this);">
</FORM>
</HTML> 

Executing a Global Search and Ignoring Case

Regular expressions have two optional flags that allow for global and case insensitive searching. To indicate a global search, use the g flag. To indicate a case insensitive search, use the i flag. These flags can be used separately or together in either order, and are included as part of the regular expression.

To include a flag with the regular expression, use this syntax:

re = /pattern/[g|i|gi]
re = new RegExp("pattern", ['g'|'i'|'gi'])
Note that the flags, i and g, are an integral part of a regular expression. They cannot be added or removed later.

For example, re = /\w+\s/g creates a regular expression that looks for one or more characters followed by a space, and it looks for this combination throughout the string.

<SCRIPT LANGUAGE="JavaScript1.2">
re = /\w+\s/g;
str = "fee fi fo fum";
myArray = str.match(re);
document.write(myArray);
</SCRIPT>
This displays ["fee ", "fi ", "fo "]. In this example, you could replace the line:

re = /\w+\s/g;
with:

re = new RegExp("\\w+\\s", "g");
and get the same result.

Examples

Changing the Order in an Input String

The following example illustrates the formation of regular expressions and the use of string.split() and string.replace().

It cleans a roughly-formatted input string containing names (first name first) separated by blanks, tabs and exactly one semicolon.

Finally, it reverses the name order (last name first) and sorts the list.

<SCRIPT LANGUAGE="JavaScript1.2">
// The name string contains multiple spaces and tabs,
// and may have multiple spaces between first and last names.
names = new String ( "Harry Trump ;Fred Barney; Helen Rigby ;\
       Bill Abel ;Chris Hand ")
document.write ("---------- Original String" + "<BR>" + "<BR>")
document.write (names + "<BR>" + "<BR>")
// Prepare two regular expression patterns and array storage.
// Split the string into array elements.
// pattern: possible white space then semicolon then possible white space
pattern = /\s*;\s*/
// Break the string into pieces separated by the pattern above and
// and store the pieces in an array called nameList
nameList = names.split (pattern)
// new pattern: one or more characters then spaces then characters.
// Use parentheses to "memorize" portions of the pattern.
// The memorized portions are referred to later.
pattern = /(\w+)\s+(\w+)/
// New array for holding names being processed.
bySurnameList = new Array;
// Display the name array and populate the new array
// with comma-separated names, last first.
//
// The replace method removes anything matching the pattern
// and replaces it with the memorized string--second memorized portion
// followed by comma space followed by first memorized portion.
//
// The variables $1 and $2 refer to the portions
// memorized while matching the pattern.
document.write ("---------- After Split by Regular Expression" + "<BR>")
for ( i = 0; i < nameList.length; i++) {
   document.write (nameList[i] + "<BR>")
   bySurnameList[i] = nameList[i].replace (pattern, "$2, $1")
}
// Display the new array.
document.write ("---------- Names Reversed" + "<BR>")
for ( i = 0; i < bySurnameList.length; i++) {
   document.write (bySurnameList[i] + "<BR>")
}
// Sort by last name, then display the sorted array.
bySurnameList.sort()
document.write ("---------- Sorted" + "<BR>")
for ( i = 0; i < bySurnameList.length; i++) {
   document.write (bySurnameList[i] + "<BR>")
}
document.write ("---------- End" + "<BR>")
</SCRIPT> 

Using Special Characters to Verify Input

In the following example, a user enters a phone number. When the user presses Enter, the script checks the validity of the number. If the number is valid (matches the character sequence specified by the regular expression), the script posts a window thanking the user and confirming the number. If the number is invalid, the script posts a window telling the user that the phone number isn't valid.

The regular expression looks for zero or one open parenthesis \(?, followed by three digits \d{3}, followed by zero or one close parenthesis \)?, followed by one dash, forward slash, or decimal point and when found, remember the character ([-\/\.]), followed by three digits \d{3}, followed by the remembered match of a dash, forward slash, or decimal point \1, followed by four digits \d{4}.

The Change event activated when the user presses Enter, sets the value of RegExp.input.

<HTML>
<SCRIPT LANGUAGE = "JavaScript1.2">
re = /\(?\d{3}\)?([-\/\.])\d{3}\1\d{4}/
function testInfo() {
   OK = re.exec()
   if (!OK)
      window.alert (RegExp.input +
         " isn't a phone number with area code!")
   else
      window.alert ("Thanks, your phone number is " + OK[0])
}
</SCRIPT>
Enter your phone number (with area code) and then press Enter.
<FORM>
<INPUT TYPE="text" NAME="Phone" onChange="testInfo(this);">
</FORM>
</HTML>


[Contents] [Previous] [Next] [Index]

Last Updated: 11/26/97 09:25:49


Copyright 1997 Netscape Communications Corporation