RegExp constructor function, the predefined RegExp object has static properties that are set whenever any regular expression is used.
| Core object | |
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Created by
A literal text format or the RegExp constructor function.
The literal format is used as follows:
/pattern/flags
The constructor function is used as follows:
new RegExp("pattern", "flags")
Parameters
pattern | The text of the regular expression. |
flags | (Optional) If specified, flags can have one of the following 3 values: |
Notice that the parameters to the literal format do not use quotation marks to indicate strings, while the parameters to the constructor function do use quotation marks. So the following expressions create the same regular expression:
/ab+c/i
new RegExp("ab+c", "i") Description
When using the constructor function, the normal string escape rules (preceding special characters with \ when included in a string) are necessary. For example, the following are equivalent:
re = new RegExp("\\w+")
Table 4.3 provides a complete list and description of the special characters that can be used in regular expressions.
re = /\w+/
Table 4.3 Special characters in regular expressions.
The literal notation provides compilation of the regular expression when the expression is evaluated. Use literal notation when the regular expression will remain constant. For example, if you use literal notation to construct a regular expression used in a loop, the regular expression won't be recompiled on each iteration.
The constructor of the regular expression object, for example, new RegExp("ab+c"), provides runtime compilation of the regular expression. Use the constructor function when you know the regular expression pattern will be changing, or you don't know the pattern and are getting it from another source, such as user input. Once you have a defined regular expression, and if the regular expression is used throughout the script and may change, you can use the compile method to compile a new regular expression for efficient reuse.
A separate predefined RegExp object is available in each window; that is, each separate thread of JavaScript execution gets its own RegExp object. Because each script runs to completion without interruption in a thread, this assures that different scripts do not overwrite values of the RegExp object.
The predefined RegExp object contains the static properties input, multiline, lastMatch, lastParen, leftContext, rightContext, and $1 through $9. The input and multiline properties can be preset. The values for the other static properties are set after execution of the exec and test methods of an individual regular expression object, and after execution of the match and replace methods of String.
Property Summary
Note that several of the RegExp properties have both long and short (Perl-like) names. Both names always refer to the same value. Perl is the programming language from which JavaScript modeled its regular expressions.
| Compiles a regular expression object. |
| Executes a search for a match in its string parameter. |
| Tests for a match in its string parameter. |
Examples
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 of the global RegExp object. Note that the RegExp object name is not be prepended to the $ properties when they are passed as the second argument to the replace method.
<SCRIPT LANGUAGE="JavaScript1.2">
This displays "Smith, John".
Example 2. In the following example,
re = /(\w+)\s(\w+)/;
str = "John Smith";
newstr=str.replace(re, "$2, $1");
document.write(newstr)
</SCRIPT>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 is prepended to the $ properties.
<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>
Properties
$1, ..., $9
Properties that contain parenthesized substring matches, if any.
| Property of |
RegExp
|
| Static, Read-only | |
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Description
Because input is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.input.
The number of possible parenthesized substrings is unlimited, but the predefined RegExp object can only hold the last nine. You can access all parenthesized substrings through the returned array's indexes.
These properties can be used in the replacement text for the String.replace method. When used this way, do not prepend them with RegExp. The example below illustrates this. When parentheses are not included in the regular expression, the script interprets $n's literally (where n is a positive integer).
Examples
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 of the global RegExp object. Note that the RegExp object name is not be prepended to the $ properties when they are passed as the second argument to the replace method.
<SCRIPT LANGUAGE="JavaScript1.2">
This displays "Smith, John".
re = /(\w+)\s(\w+)/;
str = "John Smith";
newstr=str.replace(re, "$2, $1");
document.write(newstr)
</SCRIPT>$_
See input.
global
Whether or not the "g" flag is used with the regular expression.
| Property of |
RegExp
|
| Read-only | |
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Description
global is a property of an individual regular expression object.
The value of global is true if the "g" flag was used; otherwise, false. The "g" flag indicates that the regular expression should be tested against all possible matches in a string.
You cannot change this property directly. However, calling the compile method changes the value of this property.
ignoreCase
Whether or not the "i" flag is used with the regular expression.
| Property of |
RegExp
|
| Read-only | |
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Description
ignoreCase is a property of an individual regular expression object.
The value of ignoreCase is true if the "i" flag was used; otherwise, false. The "i" flag indicates that case should be ignored while attempting a match in a string.
You cannot change this property directly. However, calling the compile method changes the value of this property.
input
The string against which a regular expression is matched. $_ is another name for the same property.
| Property of |
RegExp
|
| Static | |
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Description
Because input is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.input.
If no string argument is provided to a regular expression's exec or test methods, and if RegExp.input has a value, its value is used as the argument to that method.
The script or the browser can preset the input property. If preset and if no string argument is explicitly provided, the value of input is used as the string argument to the exec or test methods of the regular expression object. input is set by the browser in the following cases:
input property is cleared after the event handler completes.
| Property of |
RegExp
|
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Description
lastIndex is a property of an individual regular expression object.
This property is set only if the regular expression used the "g" flag to indicate a global search. The following rules apply:
$& is another name for the same property.
| Property of |
RegExp
|
| Static, Read-only | |
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Description
Because lastMatch is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.lastMatch.
lastParen
The last parenthesized substring match, if any. $+ is another name for the same property.
| Property of |
RegExp
|
| Static, Read-only | |
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Description
Because lastParen is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.lastParen.
leftContext
The substring preceding the most recent match. $` is another name for the same property.
| Property of |
RegExp
|
| Static, Read-only | |
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Description
Because leftContext is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.leftContext.
multiline
Reflects whether or not to search in strings across multiple lines. $* is another name for the same property.
| Property of |
RegExp
|
| Static | |
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Description
Because multiline is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.multiline.
The value of multiline is true if multiple lines are searched, false if searches must stop at line breaks.
The script or the browser can preset the multiline property. When an event handler is called for a TEXTAREA form element, the browser sets multiline to true. multiline is cleared after the event handler completes. This means that, if you've preset multiline to true, it is reset to false after the execution of any event handler.
rightContext
The substring following the most recent match. $' is another name for the same property.
| Property of |
RegExp
|
| Static, Read-only | |
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Description
Because rightContext is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.rightContext.
source
A read-only property that contains the text of the pattern, excluding the forward slashes and "g" or "i" flags.
| Property of |
RegExp
|
| Read-only | |
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Description
source is a property of an individual regular expression object.
You cannot change this property directly. However, calling the compile method changes the value of this property.
Methods
compile
Compiles a regular expression object during execution of a script.
| Method of |
RegExp
|
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Syntax
regexp.compile(pattern, flags)
Parameters
Description
Use the compile method to compile a regular expression created with the RegExp constructor function. This forces compilation of the regular expression once only which means the regular expression isn't compiled each time it is encountered. Use the compile method when you know the regular expression will remain constant (after getting its pattern) and will be used repeatedly throughout the script.
You can also use the compile method to change the regular expression during execution. For example, if the regular expression changes, you can use the compile method to recompile the object for more efficient repeated use.
Calling this method changes the value of the regular expression's source, global, and ignoreCase properties.
exec
Executes the search for a match in a specified string. Returns a result array.
| Method of |
RegExp
|
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Syntax
regexp.exec(str)
regexp(str) Parameters
Description
As shown in the syntax description, a regular expression's exec method call be called either directly, (with regexp.exec(str)) or indirectly (with regexp(str)).
If you are executing a match simply to find true or false, use the test method or the String search method.
If the match succeeds, the
exec method returns an array and updates properties of the regular expression object and the predefined regular expression object, RegExp. If the match fails, the exec method returns null.
Consider the following example:
<SCRIPT LANGUAGE="JavaScript1.2">
The following table shows the results for this script:
//Match one d followed by one or more b's followed by one d
//Remember matched b's and the following d
//Ignore case
myRe=/d(b+)(d)/ig;
myArray = myRe.exec("cdbBdbsbz");
</SCRIPT>
If your regular expression uses the
"g" flag, you can use the exec method multiple times to find successive matches in the same string. When you do so, the search starts at the substring of str specified by the regular expression's lastIndex property. For example, assume you have this script:
<SCRIPT LANGUAGE="JavaScript1.2">
This script displays the following text:
Found abb. Next match starts at 3
myRe=/ab*/g;
str = "abbcdefabh"
myArray = myRe.exec(str);
document.writeln("Found " + myArray[0] +
". Next match starts at " + myRe.lastIndex)
mySecondArray = myRe.exec(str);
document.writeln("Found " + mySecondArray[0] +
". Next match starts at " + myRe.lastIndex)
</SCRIPT>
Found ab. Next match starts at 9
Examples
In the following example, the user enters a name and the script executes a match against the input. It then cycles through the array to see if other names match the user's name.
This script assumes that first names of registered party attendees are preloaded into the array A, perhaps by gathering them from a party database.
<HTML>
<SCRIPT LANGUAGE="JavaScript1.2">
A = ["Frank", "Emily", "Jane", "Harry", "Nick", "Beth", "Rick",
"Terrence", "Carol", "Ann", "Terry", "Frank", "Alice", "Rick",
"Bill", "Tom", "Fiona", "Jane", "William", "Joan", "Beth"]function lookup() {
firstName = /\w+/i();
if (!firstName)
window.alert (RegExp.input + " isn't a name!");
else {
count = 0;
for (i=0; i<A.length; i++)
if (firstName[0].toLowerCase() == A[i].toLowerCase()) count++;
if (count ==1)
midstring = " other has ";
else
midstring = " others have ";
window.alert ("Thanks, " + count + midstring + "the same name!")
}
}</SCRIPT>
Enter your first name and then press Enter.
<FORM> <INPUT TYPE:"TEXT" NAME="FirstName" onChange="lookup(this);"> </FORM>
</HTML>
test
Executes the search for a match between a regular expression and a specified string. Returns true or false.
| Method of |
RegExp
|
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
Syntax
regexp.test(str)
Parameters
Description
When you want to know whether a pattern is found in a string use the test method (similar to the String.search method); for more information (but slower execution) use the exec method (similar to the String.match method).
Example
The following example prints a message which depends on the success of the test:
function testinput(re, str){
if (re.test(str))
midstring = " contains ";
else
midstring = " does not contain ";
document.write (str + midstring + re.source);
}
Last Updated: 10/31/97 12:30:31