The Jargon File (version 4.4.7, 29 Dec 2003):

lexer
 /lek'sr/, n.

    Common hacker shorthand for lexical analyzer, the input-tokenizing stage in
    the parser for a language (the part that breaks it into word-like pieces).
    ?Some C lexers get confused by the old-style compound ops like =-.?


The Free On-line Dictionary of Computing (30 December 2018):

lexical analyser
lexer

    (Or "scanner") The initial input stage of a
   language processor (e.g. a compiler), the part that performs
   lexical analysis.

   (1995-04-05)