Editing Operator-precedence parser (section)

=== Pseudocode ===

The pseudocode for the algorithm is as follows.  The parser starts at function ''parse_expression''.  Precedence levels are greater than or equal to 0.

 <u>parse_expression()</u>
     '''return''' parse_expression_1(parse_primary(), 0)

 <u>parse_expression_1(lhs, min_precedence)</u>
     ''lookahead'' := peek next token
     '''while''' ''lookahead'' is a binary operator whose precedence is >= ''min_precedence''
         ''op'' := ''lookahead''
         advance to next token
         ''rhs'' := ''parse_primary'' ()
         ''lookahead'' := peek next token
         '''while''' ''lookahead'' is a binary operator whose precedence is greater
                  than ''op''<nowiki>'</nowiki>s, or a right-associative operator
                  whose precedence is equal to ''op'''s
             ''rhs'' := ''parse_expression_1'' (''rhs'', precedence of ''op'' + (1 if ''lookahead'' precedence is greater, else 0))
             ''lookahead'' := peek next token
         ''lhs'' := the result of applying ''op'' with operands ''lhs'' and ''rhs''
     '''return''' ''lhs''

Note that in the case of a production rule like this (where the operator can only appear once):

<syntaxhighlight lang="abnf">
equality-expression ::= additive-expression  ( '==' | '!=' ) additive-expression
</syntaxhighlight>

the algorithm must be modified to accept only binary operators whose precedence is > ''min_precedence''.