Simple Single-character Patterns are :
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Single explicit character e.g. a
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dot . which matches ANY character except newline \n
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Character class is a Single-character Pattern and represented as a set [c1c2c3...cN] which matches any one of the listed characters
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[ABCDE] matches A B C D or E
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[0-9] is same as [0123456789]
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[a-zA-Z] matches any lower or upper case letter
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Negated character class is represented by a carat ^ after left [ square bracket
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[^0-9] matches any character which is NOT a digit 0 1 2 3 4 5 6 7 8 9 (there is another critical use of ^ -- see later)
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There are a set of special character classes shown overleaf such as \w which is equivalent to [A-Za-z0-9_].
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