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144 ” PCRE Extension i POSIX Extended Regular Expressions Many PHP developers will cut their teeth on regular expressions using the POSIX regular expression extension, also called the ereg extension. The functions from this extension are being deprecated in PHP 5.3 in favor of those in the PCRE extension, which are faster and provide a more powerful feature set. Aside from differences in syntax for some special character ranges, most ereg expressions require only the addition of expression delimiters to work with preg functions. P a t t e r nBasics Let’s start with something simple: detection of a substring anywhere within a string. N o t i c e that the pattern in the preg_match() call is fairly similar to the string used in the strpos() call. In the former, / is used on either side of the pattern to indicate its beginning and end. The first character in the pattern string is considered to be the pattern delimiter and can be any character you specify. When choosing what you want to use for this character (/ is the most common choice), bear in mind that you will have to escape it (covered in the Escaping section later) if you use it within the pattern. This will make more sense a little later in the chapter. A difference between the two functions used in this example is that strpos() returns the location of the substring within the string beginning at 0 or false if the substring is not contained within the string. This requires the use of the === operator to tell the difference between the substring being matched at the beginning of the string or not at all. By contrast, preg_match() returns the number of matches it found. This will be either 0 or 1 since preg_match() stops searching once it finds a match.
PCRE Extension ” 145 Anchors Y o umay want to check for the presence of a pattern at the beginning or end of a string rather than simply checking to see if the pattern is contained anywhere within the string. The meta-characters for this are collectively referred to as anchors. • ˆ (also called the circumflex character) is used at the beginning of an expression within a pattern to indicate that matching should start at the beginning of a string. • Likewise, $ is used to indicate that matching of an expression within a pattern should stop at the end of a string. • When used together, ˆ and $ can indicate that the entirety of $string matches the pattern exactly. i St a r t of St r i n g or Line It’s important to note that the behavior of these two operators can vary. By default, they match the beginning and end of $string. If the multi-line modifier is used, they match the beginning and end of each line in $string instead. This will be covered in more detail later in the Mo d i fi e r s section of this chapter.
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