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[ < ] [ > ] [ << ] [ Up ] [ >> ] [ Top ] [ Contents ] [ Index ] [ ? ] 5. Regular Expressions A regular expression is a pattern that describes a set of strings. Regular expressions are constructed analogously to arithmetic expressions, by using various operators to combine smaller expressions. grep understands two different versions of regular expression syntax: "basic"(BRE) and "extended"(ERE). In GNU grep , there is no difference in available functionality using either syntax. In other implementations, basic regular expressions are less powerful. The following description applies to extended regular expressions; differences for basic regular expressions are summarized afterwards. The fundamental building blocks are the regular expressions that match a single character. Most characters, including all letters and digits, are regular expressions that match themselves. Any metacharacter with special meaning may be quoted by preceding it with a backslash. A regular expression may be followed by one of several repetition operators: ` . ' The period ` . ' matches any single character. ` ? ' The preceding item is optional and will be matched at most once. ` * ' The preceding item will be matched zero or more times. ` + ' The preceding item will be matched one or more times. ` { n } ' The preceding item is matched exactly n times. ` { n ,} ' The preceding item is matched n or more times. ` { n , m } ' The preceding item is matched at least n times, but not more than m times. Two regular expressions may be concatenated; the resulting regular expression matches any string formed by concatenating two substrings that respectively match the concatenated subexpressions. Two regular expressions may be joined by the infix operator ` | '; the resulting regular expression matches any string matching either subexpression. Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole
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