字符匹配
正則表達式的關鍵之處在于確定你要搜索匹配的東西，如果沒有這一概念，Res將毫無用處。
每一個表達式都包含需要查找的指令，如表A所示。
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Table A: Character-matching regular expressions
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操作
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解釋
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例子
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結果
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.
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Match any one character
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grep .ord sample.txt
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Will match “ford”, “lord”, “2ord”, etc. in the file sample.txt.
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[ ]
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Match any one character listed between the brackets
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grep [cng]ord sample.txt
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Will match only “cord”, “nord”, and “gord”
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[^ ]
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Match any one character not listed between the brackets
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grep [^cn]ord sample.txt
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Will match “lord”, “2ord”, etc. but not “cord” or “nord”
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grep [a-zA-Z]ord sample.txt
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Will match “aord”, “bord”, “Aord”, “Bord”, etc.
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grep [^0-9]ord sample.txt
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Will match “Aord”, “aord”, etc. but not “2ord”, etc.
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重復操作符
重復操作符，或數量詞，都描述了查找一個特定字符的次數。它們常被用于字符匹配語法以查找多行的字符，可參見表B。
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Table B: Regular expression repetition operators
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操作
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解釋
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例子
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結果
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?
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Match any character one time, if it exists
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egrep “?erd” sample.txt
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Will match “berd”, “herd”, etc. and “erd”
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*
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Match declared element multiple times, if it exists
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egrep “n.*rd” sample.txt
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Will match “nerd”, “nrd”, “neard”, etc.
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+
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Match declared element one or more times
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egrep “[n]+erd” sample.txt
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Will match “nerd”, “nnerd”, etc., but not “erd”
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{n}
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Match declared element exactly n times
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egrep “[a-z]{2}erd” sample.txt
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Will match “cherd”, “blerd”, etc. but not “nerd”, “erd”, “buzzerd”, etc.
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{n,}
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Match declared element at least n times
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egrep “.{2,}erd” sample.txt
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Will match “cherd” and “buzzerd”, but not “nerd”
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{n,N}
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Match declared element at least n times, but not more than N times
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egrep “n[e]{1,2}rd” sample.txt
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Will match “nerd” and “neerd”
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posted on 2006-10-25 16:58
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