RegExp constructor function, the predefined RegExp object has static properties that are set whenever any regular expression is used.
JavaScript 1.3: added | |
RegExp constructor function.
The literal format is used as follows:
/pattern/flagsThe constructor function is used as follows:
new RegExp("pattern"[, "flags"])
pattern | |
flags |
/ab+c/i
new RegExp("ab+c", "i")
re = new RegExp("\\w+")
re = /\w+/
The following table provides a complete list and description of the special characters that can be used in regular expressions.
Table 1.1 Special characters in regular expressions.
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.
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.
watch and unwatch methods from Object.
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>
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).
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>
input.
multiline.
lastMatch.
lastParen.
leftContext.
rightContext.
regexp.compile(pattern[, flags])
regexp |
|
pattern | |
flags |
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.
Object.constructor.
regexp.exec([str])
regexp([str])
regexp |
|
str |
The string against which to match the regular expression. If omitted, the value of |
exec method can 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>
| Object | Property/Index | Description |
Example
myArray
| index
| input
| [0]
| [1], ...[n]
| myRe lastIndex
| ignoreCase
| global
| source
| RegExp lastMatch
| leftContext
| rightContext
| $1, ...$9
| lastParen
| |
|---|
"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
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>
abb. Next match starts at 3<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>
"g" flag is used with the regular expression. 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.
"i" flag is used with the regular expression. 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.
$_ is another name for the same property. 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:
TEXT form element, input is set to the value of the contained text. TEXTAREA form element, input is set to the value of the contained text. Note that multiline is also set to true so that the match can be executed over the multiple lines of text. SELECT form element, input is set to the value of the selected text. Link object, input is set to the value of the text between <A HREF=...> and </A>. input property is cleared after the event handler completes.
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:
lastIndex is greater than the length of the string, regexp.test and regexp.exec fail, and lastIndex is set to 0.lastIndex is equal to the length of the string and if the regular expression matches the empty string, then the regular expression matches input starting at lastIndex. lastIndex is equal to the length of the string and if the regular expression does not match the empty string, then the regular expression mismatches input, and lastIndex is reset to 0. lastIndex is set to the next position following the most recent match.
re = /(hi)?/g | |
re("hi") | |
re("hi") |
$& is another name for the same property.lastMatch is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.lastMatch.
$+ is another name for the same property.lastParen is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.lastParen.
$` is another name for the same property.leftContext is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.leftContext.
multiline is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.multiline.
The value of multiline is true if multiple lines are searched, false if searches must stop at line breaks.
The script or the browser can preset the multiline property. When an event handler is called for a TEXTAREA form element, the browser sets multiline to true. multiline is cleared after the event handler completes. This means that, if you've preset multiline to true, it is reset to false after the execution of any event handler.
Function.prototype.$' is another name for the same property.rightContext is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.rightContext.
"g" or "i" flags.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.
true or false.regexp.test([str])
regexp |
|
str |
The string against which to match the regular expression. If omitted, the value of |
test method (similar to the String.search method); for more information (but slower execution) use the exec method (similar to the String.match method).
function testinput(re, str){
if (re.test(str))
midstring = " contains ";
else
midstring = " does not contain ";
document.write (str + midstring + re.source);
}
toSource()
toSource method returns the following values:
function Boolean() {
[native code]
}RegExp, toSource returns a string representing the source code.
Object.toSource
toString()
RegExp object overrides the toString method of the Object object; it does not inherit Object.toString. For RegExp objects, the toString method returns a string representation of the object.
myExp = new RegExp("a+b+c");
alert(myExp.toString()) displays "/a+b+c/"
Object.toString
valueOf()
valueOf method of RegExp returns the primitive value of a RegExp object as a string data type. This value is equivalent to RegExp.toString.
This method is usually called internally by JavaScript and not explicitly in code.
myExp = new RegExp("a+b+c");
alert(myExp.valueOf()) displays "/a+b+c/"
RegExp.toString, Object.valueOf
Last Updated: 10/29/98 20:17:27