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Epsilon Parser (eps)

The Epsilon (eps) is a multi-purpose parser that returns a zero length match.

Simple Form

In its simplest form, eps matches the null string and always returns a match of zero length:

eps // always returns a zero-length match

This form is usually used to trigger a semantic action unconditionally. For example, it is useful in triggering error messages when a set of alternatives fail:

r = a | b | c | eps[error()]; // Call error if a, b, and c fail to match
Semantic Predicate

Semantic predicates allow you to attach a conditional function anywhere in the grammar. In this role, the epsilon takes a Lazy Argument that returns true or false. The Lazy Argument is typically a test that is called to resolve ambiguity in the grammar. A parse failure will be reported when the Lazy Argument result evaluates to false. Otherwise an empty match will be reported. The general form is:

eps(f) >> rest;

The Lazy Argument f is called to do a semantic test (say, checking if a symbol is in the symbol table). If test returns true, rest will be evaluated. Otherwise, the production will return early with a no-match without ever touching rest.

// forwards to <boost/spirit/home/qi/auxiliary/eps.hpp>
#include <boost/spirit/include/qi_eps.hpp>

Also, see Include Structure.



boost::spirit::eps // alias: boost::spirit::qi::eps

Model of




A Lazy Argument that evaluates bool.

Expression Semantics

Semantics of an expression is defined only where it differs from, or is not defined in PrimitiveParser.




Match an empty string (always matches).


If f evaluates to true, return a zero length match.







For plain (eps) the complexity is O(1). For Semantic predicates (eps(f)) the complexity is defined by the function f.

[Note] Note

The test harness for the example(s) below is presented in the Basics Examples section.

Some using declarations:

using boost::spirit::qi::eps;
using boost::spirit::qi::int_;
using boost::spirit::qi::_1;
namespace phx = boost::phoenix;

Basic eps:

test_parser("", eps); // always matches

This example simulates the "classic" if_p parser. Here, int_ will be tried only if the condition, c, is true.

bool c = true; // a flag
test_parser("1234", eps(phx::ref(c) == true) >> int_);

This example simulates the "classic" while_p parser. Here, the kleene loop will exit once the condition, c, becomes true. Notice that the condition, c, is turned to false when we get to parse 4.

test_phrase_parser("1 2 3 4",
    *(eps(phx::ref(c) == true) >> int_[phx::ref(c) = (_1 == 4)]));