It contains strings and expressions to be matched by the yylex subroutine, and C commands to execute when a match is made. The lex command does not recognize your rules without this delimiter. In this section, the left column contains the pattern in the form of an extended regular expression, which will be recognized in an input file to the yylex subroutine. The right column contains the C program fragment executed when that pattern is recognized, called an action . When the lexical analyzer finds a match for the extended regular expression, the lexical analyzer executes the action associated with that extended regular expression.
Patterns can include extended characters.
If multibyte locales are installed on your system, patterns can also include multibyte characters that are part of the installed code set. The columns are separated by a tab or blanks. Each pattern can have a corresponding action, that is, a C command to execute when the pattern is matched. Without a specified action for a pattern match, the lexical analyzer copies the input pattern to the output without changing it. Reads a multibyte character from yyin .