Letter : L = a + b + c + ................... + A + B + ................... + Z
Digit : d = 0 + 1 + 2 + ..................................+9
Digit : d = 0 + 1 + 2 + ..................................+9
An Identifier in the C programming language is any string of length 1 or more that contains only letter, digit and underscore ( _ ) and begin with a letter or underscore. Therefore the regular expression for the language of all C identifier is
(L + _ ) (L+d+_)*
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