Flags and properties

The g (global) flag

When the g ( global ) flag is not included, a regular expression matches no more than one match. For example, with the g flag not included in the regular expression, the String.match() method returns only one matching substring:

var str:String = "she sells seashells by the seashore."; 
var pattern:RegExp = /sh\w*/; 
trace(str.match(pattern)) // output: she

When the g flag is set, the Sting.match() method returns multiple matches, as follows:

var str:String = "she sells seashells by the seashore."; 
var pattern:RegExp = /sh\w*/g; 
// The same pattern, but this time the g flag IS set. 
trace(str.match(pattern)); // output: she,shells,shore

The i (ignoreCase) flag

By default, regular expression matches are case-sensitive. When you set the i ( ignoreCase ) flag, case sensitivity is ignored. For example, the lowercase s in the regular expression does not match the uppercase letter S , the first character of the string:

var str:String = "She sells seashells by the seashore."; 
trace(str.search(/sh/)); // output: 13 -- Not the first character

With the i flag set, however, the regular expression does match the capital letter S :

var str:String = "She sells seashells by the seashore."; 
trace(str.search(/sh/i)); // output: 0

The i flag ignores case sensitivity only for the A – Z and a – z characters, but not for extended characters such as É and é .

The m (multiline) flag

If the m ( multiline ) flag is not set, the ^ matches the beginning of the string and the $ matches the end of the string. If the m flag is set, these characters match the beginning of a line and end of a line, respectively. Consider the following string, which includes a newline character:

var str:String = "Test\n"; 
str += "Multiline"; 
trace(str.match(/^\w*/g)); // Match a word at the beginning of the string.

Even though the g ( global ) flag is set in the regular expression, the match() method matches only one substring, since there is only one match for the ^ —the beginning of the string. The output is:

Test

Here is the same code with the m flag set:

var str:String = "Test\n"; 
str += "Multiline"; 
trace(str.match(/^\w*/gm)); // Match a word at the beginning of lines. 

This time, the output includes the words at the beginning of both lines:

Test,Multiline

Note that only the \n character signals the end of a line. The following characters do not:

• Return ( \r ) character

• Unicode line-separator ( \u2028 ) character

• Unicode paragraph-separator ( \u2029 ) character

The s (dotall) flag

If the s ( dotall or “dot all”) flag is not set, a dot ( . ) in a regular expression pattern does not match a newline character ( \n ). So for the following example, there is no match:

var str:String = "<p>Test\n"; 
str += "Multiline</p>"; 
var re:RegExp = /<p>.*?<\/p>/; 
trace(str.match(re)); 

However, if the s flag is set, the dot matches the newline character:

var str:String = "<p>Test\n"; 
str += "Multiline</p>"; 
var re:RegExp = /<p>.*?<\/p>/s; 
trace(str.match(re)); 

In this case, the match is the entire substring within the <p> tags, including the newline character:

<p>Test 
Multiline</p>

The x (extended) flag

Regular expressions can be difficult to read, especially when they include a lot of metasymbols and metasequences. For example:

/<p(>|(\s*[^>]*>)).*?<\/p>/gi

When you use the x ( extended ) flag in a regular expression, any blank spaces that you type in the pattern are ignored. For example, the following regular expression is identical to the previous example:

/     <p    (>  | (\s* [^>]* >))    .*?    <\/p>  /gix

If you have the x flag set and do want to match a blank space character, precede the blank space with a backslash. For example, the following two regular expressions are equivalent:

/foo bar/ 
/foo \ bar/x

The lastIndex property

The lastIndex property specifies the index position in the string at which to start the next search. This property affects the exec() and test() methods called on a regular expression that has the g flag set to true . For example, consider the following code:

var pattern:RegExp = /p\w*/gi; 
var str:String = "Pedro Piper picked a peck of pickled peppers."; 
trace(pattern.lastIndex); 
var result:Object = pattern.exec(str); 
while (result != null) 
{ 
    trace(pattern.lastIndex); 
    result = pattern.exec(str); 
}

The lastIndex property is set to 0 by default (to start searches at the beginning of the string). After each match, it is set to the index position following the match. Therefore, the output for the preceding code is the following:

0 
5 
11 
18 
25 
36 
44

If the global flag is set to false , the exec() and test() methods do not use or set the lastIndex property.

The match() , replace() , and search() methods of the String class start all searches from the beginning of the string, regardless of the setting of the lastIndex property of the regular expression used in the call to the method. (However, the match() method does set lastIndex to 0.)

You can set the lastIndex property to adjust the starting position in the string for regular expression matching.

The source property

The source property specifies the string that defines the pattern portion of a regular expression. For example:

var pattern:RegExp = /foo/gi; 
trace(pattern.source); // foo