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146 ” PCRE Extension Alternation It’s possible to check for multiple expressions simultaneously in a single pattern, also called alternation, using the pipe meta-character |. N o t e that the ˆ and $ are not implicitly applied to all expressions in an alternation; they must be used explicitly for each expression. The first example returns 1 because ’abar’ contains ’bar’ since , ˆ is not applied to that expression in the pattern. The second example does apply ˆ to ’bar’ and, since ’abar’ begins with neither ’foo’ nor ’bar’ it returns , 0. R e p e t i t i o n and Quantifiers P a r t of a pattern may or may not be present, or may be repeated a number of times. This is referred to as repetition and involves using meta-characters collectively referred to as quantifiers.
PCRE Extension ” 147 // Matches ’a’ 1 or more times, however many that may be $matches = (preg_match(’/a+/’, $string) == 1); // Matches ’a’ 0 times or 1 time if present, same as ? $matches = (preg_match(’/a{0,1}/’, $string) == 1); // Matches ’a’ 0 or more times, same as * $matches = (preg_match(’/a{0,}/’, $string) == 1); // Matches ’a’ 1 or more times, same as + $matches = (preg_match(’/a{1,}/’, $string) == 1); // Matches ’a’ exactly 2 times $matches = (preg_match(’/a{2}/’, $string) == 1); ?> N o t e that any use of curly brackets that is not of the form {X}, {X,}, or {X,Y} will be treated as a literal string within the pattern. S u b p a t t e r n s Y o u notice ’ l inl the examples from the previous section that only a single character was used. This is because the concept of subpatterns hadn’t been introduced yet. T ounderstand these, it’s best to look an example that doesn’t use them in order to understand the effect they have on how the pattern matches. Without subpatterns there would be no way to match, for example, one or more instances of the string ’ab’ Su b . p a t t e r n s solve this pattern by allowing individuals parts of a pattern to be grouped using parentheses.
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Chapter 1 Introduction If you are l
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Introduction ” 3 in some instance
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