Lexers commonly use regular expressions to identify tokens by matching character patterns defined in the lexical grammar.
Consider the regular expression [a-zA-Z_][a-zA-Z0-9_]*, which matches identifiers in many programming languages. This pattern describes a sequence that starts with a letter or underscore, followed by any number of letters, digits, or underscores. Tools like Lex or Flex use regular expressions to automate the generation of lexical analyzers, which handle the task of tokenizing source code according to the patterns defined by the developer.