The Identifier of Programming Language C

Letter :  L =  a + b + c + ................... + A + B + ................... + Z
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+_)*