Syntactic Recognition

Treetop grammars are written in a custom language based on parsing expression grammars. Literature on the subject of parsing expression grammars (PEGs) is useful in writing Treetop grammars.

PEGs have no separate lexical analyser (since the algorithm has the same time-complexity guarantees as the best lexical analysers) so all whitespace and other lexical niceties (like comments) must be explicitly handled in the grammar. A further benefit is that multiple PEG grammars may be seamlessly composed into a single parser.

Grammar Structure

Treetop grammars look like this:

require "my_stuff"

grammar GrammarName
  include Module::Submodule

  rule rule_name

  rule rule_name


The main keywords are:

  • grammar : This introduces a new grammar. It is followed by a constant name to which the grammar will be bound when it is loaded.

  • include: This causes the generated parser to include the referenced Ruby module (which may be another parser)

  • require: This must be at the start of the file, and is passed through to the emitted Ruby grammar

  • rule : This defines a parsing rule within the grammar. It is followed by a name by which this rule can be referenced within other rules. It is then followed by a parsing expression defining the rule.

A grammar may be surrounded by one or more nested module statements, which provides a namespace for the generated Ruby parser.

Treetop will emit a module called GrammarName and a parser class called GrammarNameParser (in the module namespace, if specified).

Parsing Expressions

Each rule associates a name with a parsing expression. Parsing expressions are a generalization of vanilla regular expressions. Their key feature is the ability to reference other expressions in the grammar by name.

Terminal Symbols


Strings are surrounded in double or single quotes and must be matched exactly.

  • "foo"
  • 'foo'

Character Classes

Character classes are surrounded by brackets. Their semantics are identical to those used in Ruby's regular expressions.

  • [a-zA-Z]
  • [0-9]

The Anything Symbol

The anything symbol is represented by a dot (.) and matches any single character.


An empty string matches at any position and consumes no input. It's useful when you wish to treat a single symbol as part of a sequence, for example when an alternate rule will be processed using shared code.

    rule alts
      ( foo bar / baz '' )
        def value
{|e| e.text_value }

Nonterminal Symbols

Nonterminal symbols are unquoted references to other named rules. They are equivalent to an inline substitution of the named expression.

rule foo
  "the dog " bar

rule bar

The above grammar is equivalent to:

rule foo
  "the dog jumped"

Ordered Choice

Parsers attempt to match ordered choices in left-to-right order, and stop after the first successful match.

"foobar" / "foo" / "bar"

Note that if "foo" in the above expression came first, "foobar" would never be matched. Note also that the above rule will match "bar" as a prefix of "barbie". Care is required when it's desired to match language keywords exactly.


Sequences are a space-separated list of parsing expressions. They have higher precedence than choices, so choices must be parenthesized to be used as the elements of a sequence.

"foo" "bar" ("baz" / "bop")

Zero or More

Parsers will greedily match an expression zero or more times if it is followed by the star (*) symbol.

  • 'foo'* matches the empty string, "foo", "foofoo", etc.

One or More

Parsers will greedily match an expression one or more times if it is followed by the plus (+) symbol.

  • 'foo'+ does not match the empty string, but matches "foo", "foofoo", etc.

Optional Expressions

An expression can be declared optional by following it with a question mark (?).

  • 'foo'? matches "foo" or the empty string.

Repetition count

A generalised repetition count (minimum, maximum) is also available.

  • 'foo' 2.. matches 'foo' two or more times
  • 'foo' 3..5 matches 'foo' from three to five times
  • 'foo' ..4 matches 'foo' from zero to four times

Lookahead Assertions

Lookahead assertions can be used to make parsing expressions context-sensitive. The parser will look ahead into the buffer and attempt to match an expression without consuming input.

Positive Lookahead Assertion

Preceding an expression with an ampersand (&) indicates that it must match, but no input will be consumed in the process of determining whether this is true.

  • "foo" &"bar" matches "foobar" but only consumes up to the end "foo". It will not match "foobaz".

Negative Lookahead Assertion

Preceding an expression with a bang (!) indicates that the expression must not match, but no input will be consumed in the process of determining whether this is true.

  • "foo" !"bar" matches "foobaz" but only consumes up to the end "foo". It will not match "foobar".

Note that a lookahead assertion may be used on any rule, not just a string terminal.

rule things
  thing (!(disallowed / ',') following)*

Here's a common use case:

rule word

rule conjunction
  primitive ('and' ' '+ primitive)*

rule primitive
  (!'and' word ' '+)*

Here's the easiest way to handle C-style comments:

rule c_comment
    (. / "\n")

Semantic predicates (positive and negative)

Sometimes you must execute Ruby code during parsing in order to decide how to proceed. This is an advanced feature, and must be used with great care, because it can change the way a Treetop parser backtracks in a way that breaks the parsing algorithm. See the notes below on how to use this feature safely.

The code block is the body of a Ruby lambda block, and should return true or false, to cause this parse rule to continue or fail (for positive sempreds), fail or continue (for negative sempreds).

  • &{ ... } Evaluate the block and fail this rule if the result is false or nil
  • !{ ... } Evaluate the block and fail this rule if the result is not false or nil

The lambda is passed a single argument which is the array of syntax nodes matched so far in the current sequence. Note that because the current rule has not yet succeeded, no syntax node has yet been constructed, and so the lambda block is being run in a context where the names of the preceding rules (or as assigned by labels) are not available to access the sub-rules.

rule id
    def is_reserved
      ReservedSymbols.include? text_value

rule foo_rule
  foo id &{|seq| seq[1].is_reserved } baz

Match "foo id baz" only if id.is_reserved. Note that id cannot be referenced by name from foo_rule, since that rule has not yet succeeded, but id has completed and so its added methods are available.

rule test_it
  foo bar &{|s| debugger; true } baz

Match foo then bar, stop to enter the debugger (make sure you have said require "ruby-debug" somewhere), then continue by trying to match baz.

Treetop, like other PEG parsers, achieves its performance guarantee by remembering which rules it has tried at which locations in the input, and what the result was. This process, called memoization, requires that the rule would produce the same result (if run again) as it produced the first time when the result was remembered. If you violate this principle in your semantic predicates, be prepared to fight Cerberus before you're allowed out of Hades again.