Passage: Tokens are defined often by regular expressions, which are understood by a lexical analyzer generator such as lex. The lexical analyzer (generated automatically by a tool like lex, or hand-crafted) reads in a stream of characters, identifies the lexemes in the stream, and categorizes them into tokens. This is termed tokenizing. If the lexer finds an invalid token, it will report an error.

Question: What will it find if it reports an error?
an invalid token