Page 599 - Beginning PHP 5.3

Chapter 18: String Matching with Regular Expressions

Modifier Description
i Causes the matching to be case insensitive: letters in the pattern match both upper -
and lowercase characters in the string
m Causes the target string to be treated as separate lines of text if it contains newlines.
This means that ^ and $ characters in the expression match not only the beginning
and end of the string, but also the beginning and end of each line in the string
s Normally, the dot ( . ) character in an expression matches any character except
newline characters. By adding this modifier you can make the dot character match
newlines too
x This modifier causes whitespace characters in the pattern to be ignored, rather than
treated as characters to match. (However, whitespace inside a character class is
never ignored.) This allows you to split your regular expression over lines and
indent it, much like regular PHP code, to aid readability. You can also include
comments in the expression by preceding them with a # symbol. If you explicitly
want to match whitespace characters when using this modifier, use “ \ “ (for a
space), “ \t ” (for a tab), or “ \s ” (for any whitespace character)

e Only used by preg_replace() . This modifier allows you to use PHP code in your
replacement string. Any backreferences ( $1 , $2 , and so on) in the replacement
string are first replaced by their matched text. Then the string is evaluated as PHP
code, and the resulting expression used for the replacement
U Inverts the “ greediness ” of quantifiers within the expression: any non - greedy
quantifiers become greedy, and any greedy quantifiers become non - greedy

For example, you can make an expression case insensitive by adding i after the closing delimiter of the
expression:

$text = “Hello, world!”;
echo preg_match( “/hello/”, $text ) . “ < br / > ”; // Displays “0”

echo preg_match( “/hello/i”, $text ) . “ < br / > ”; // Displays “1”
The following example shows how the m modifier works. The first expression attempts to match the
characters “ world! ” followed by the end of the string. Because “ world! ” is not at the end of the target
string, the match fails. However, the second expression uses the m modifier. This causes the $ character to
match the newline after “ world! ” :

$text = “Hello, world!\nHow are you today?\n”;
echo preg_match( “/world!$/”, $text ) . “ < br / > ”; // Displays “0”
echo preg_match( “/world!$/m”, $text ) . “ < br / > ”; // Displays “1”

The m modifier is useful if you ’ re working with a multiline string (such as that read from a file or
database query) that you want to treat as multiple lines of text rather than as one long string.

By adding the x modifier to your expression you can split the expression over multiple lines and add
comments — very handy for complex expressions:

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