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152 ” PCRE Extension Ranges are respective to ASCII (American St a n d a rd Code for Information Interchange). In other words, the ASCII value for the beginning character must precede the ASCII value for the ending character. Otherwise, the warning “Warning: preg_match(): Compilation failed: range out of order in character class at offset n” is emitted, where n is character offset within the regular expression. Within square brackets, single characters and special ranges are simply listed side by side with no delimiter, as shown in the second example above. Additionally, the escape sequences mentioned earlier such as \w can be used both inside and outside square brackets. i ASCII Ranges F o r an excellent ASCII lookup table, see http://www.asciitable.com. There are two other noteworthy points about character ranges, as illustrated in the examples below. • T ouse a literal ] character in a character range, escape it in the same manner in which other meta-characters are escaped. • T onegate a character range, use ˆ as the first character in that character range. (Yes,this can be confusing since ˆ is also used to denote the beginning of a line or entire string when it is not used inside a character range.) N o t e that negation applies to all characters in the range. In other words, a negated character range means “ a n ycharacter that is not any of these characters.”
PCRE Extension ” 153 • T ouse a literal ˆ character in a character range, either escape it in the same manner in which other meta-characters are escaped or do not use it as the first or only character in the range. i ctype Extension Some simple patterns have equivalent functions available in the ctype library. These generally perform better and should be used ov e r PCRE when appropriate. See http://php.net/ctype for more information on the ctype extension and the functions it offers. M o d i fi e r s The reason for having pattern delimiters to denote the start and end of a pattern is that the pattern precedes modifiers that affect the matching behavior of metacharacters. H e r e are a few modifiers that may prove useful in web scraping applications. • i: Any letters in the pattern will match both uppercase and lowercase regardless of the case of the letter used in the pattern. • m: ˆ and $ will match the beginning and ends of lines within the string (delimited by line feed characters) rather than the beginning and end of the entire string. • s (lowercase): The . meta-character will match line feeds, which it does not by default. • S (uppercase): Additional time will be spent to analyze the pattern in order to speed up subsequent matches with that pattern. U s e f u lfor patterns used multiple times. • U: By default, the quantifiers * and + behave in a manner referred to as “ g r e e d y . ” That is, they match as many characters as possible rather than as few as possible. This modifier forces the latter behavior.
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