Why are regular expression used in lexical analysis instead of grammars?

Question

Even though grammars can do what regular expressions do, why are regular expression employed instead of grammars in lexical analysis?

Answer 1

Regular languages are faster to recognize, and so they are preferred whenever possible. Fortunately, regular expressions are usually good enough for describing the lexemes of a language.

Indeed, in some sense the separation between lexical analysis and parsing is exactly due to this fact, that lexical analysis can be done more efficiently than parsing.