A long-lived project that still receives updates
A Ruby parser written in pure Ruby.


>= 1.15, < 3.0.0
~> 0.3.2
~> 5.10
~> 13.0.1
~> 0.15.1
>= 0


~> 2.4.1
>= 0
 Project Readme


Gem Version Tests

Parser is a production-ready Ruby parser written in pure Ruby. It recognizes as much or more code than Ripper, Melbourne, JRubyParser or ruby_parser, and is vastly more convenient to use.

You can also use unparser to produce equivalent source code from Parser's ASTs.

Sponsored by Evil Martians. MacRuby and RubyMotion support sponsored by CodeClimate.


$ gem install parser


Load Parser (see the backwards compatibility section below for explanation of emit_* calls):

require 'parser/current'
# opt-in to most recent AST format:
Parser::Builders::Default.emit_lambda              = true
Parser::Builders::Default.emit_procarg0            = true
Parser::Builders::Default.emit_encoding            = true
Parser::Builders::Default.emit_index               = true
Parser::Builders::Default.emit_arg_inside_procarg0 = true
Parser::Builders::Default.emit_forward_arg         = true
Parser::Builders::Default.emit_kwargs              = true
Parser::Builders::Default.emit_match_pattern       = true

Parse a chunk of code:

p Parser::CurrentRuby.parse("2 + 2")
# (send
#   (int 2) :+
#   (int 2))

Access the AST's source map:

p Parser::CurrentRuby.parse("2 + 2").loc
# #<Parser::Source::Map::Send:0x007fe5a1ac2388
#   @dot=nil,
#   @begin=nil,
#   @end=nil,
#   @selector=#<Source::Range (string) 2...3>,
#   @expression=#<Source::Range (string) 0...5>>

p Parser::CurrentRuby.parse("2 + 2").loc.selector.source
# "+"

Traverse the AST: see the documentation for gem ast.

Parse a chunk of code and display all diagnostics:

parser =
parser.diagnostics.consumer = lambda do |diag|
  puts diag.render

buffer ='(string)', source: "foo *bar")

p parser.parse(buffer)
# (string):1:5: warning: `*' interpreted as argument prefix
# foo *bar
#     ^
# (send nil :foo
#   (splat
#     (send nil :bar)))

If you reuse the same parser object for multiple #parse runs, you need to #reset it.

You can also use the ruby-parse utility (it's bundled with the gem) to play with Parser:

$ ruby-parse -L -e "2+2"
  (int 2) :+
  (int 2))
 ~ selector
~~~ expression
(int 2)
~ expression
(int 2)

$ ruby-parse -E -e "2+2"
^ tINTEGER 2                                    expr_end     [0 <= cond] [0 <= cmdarg]
 ^ tPLUS "+"                                    expr_beg     [0 <= cond] [0 <= cmdarg]
  ^ tINTEGER 2                                  expr_end     [0 <= cond] [0 <= cmdarg]
  ^ false "$eof"                                expr_end     [0 <= cond] [0 <= cmdarg]
  (int 2) :+
  (int 2))


  • Precise source location reporting.
  • Documented AST format which is convenient to work with.
  • A simple interface and a powerful, tweakable one.
  • Parses 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 3.0, 3.1, and 3.2 syntax with backwards-compatible AST formats.
  • Parses MacRuby and RubyMotion syntax extensions.
  • Rewriting support.
  • Parsing error recovery.
  • Improved clang-like diagnostic messages with location information.
  • Written in pure Ruby, runs on MRI >=2.0.0, JRuby and Rubinius (and historically, all versions of Ruby since 1.8)
  • Only one runtime dependency: the ast gem.
  • Insane Ruby lexer rewritten from scratch in Ragel.
  • 100% test coverage for Bison grammars (except error recovery).
  • Readable, commented source code.


Documentation for Parser is available online.

Node names

Several Parser nodes seem to be confusing enough to warrant a dedicated README section.


The (block) node passes a Ruby block, that is, a closure, to a method call represented by its first child, a (send), (super) or (zsuper) node. To demonstrate:

$ ruby-parse -e 'foo { |x| x + 2 }'
  (send nil :foo)
    (arg :x))
    (lvar :x) :+
    (int 2)))

(begin) and (kwbegin)

TL;DR: Unless you perform rewriting, treat (begin) and (kwbegin) as the same node type.

Both (begin) and (kwbegin) nodes represent compound statements, that is, several expressions which are executed sequentally and the value of the last one is the value of entire compound statement. They may take several forms in the source code:

  • foo; bar: without delimiters
  • (foo; bar): parenthesized
  • begin foo; bar; end: grouped with begin keyword
  • def x; foo; bar; end: grouped inside a method definition

and so on.

$ ruby-parse -e '(foo; bar)'
  (send nil :foo)
  (send nil :bar))
$ ruby-parse -e 'def x; foo; bar end'
(def :x
    (send nil :foo)
    (send nil :bar)))

Note that, despite its name, kwbegin node only has tangential relation to the begin keyword. Normally, Parser AST is semantic, that is, if two constructs look differently but behave identically, they get parsed to the same node. However, there exists a peculiar construct called post-loop in Ruby:

end while condition

This specific syntactic construct, that is, keyword begin..end block followed by a postfix while, behaves very unlike other similar constructs, e.g. (body) while condition. While the body itself is wrapped into a while-post node, Parser also supports rewriting, and in that context it is important to not accidentally convert one kind of loop into another.

$ ruby-parse -e 'begin foo end while cond'
  (send nil :cond)
    (send nil :foo)))
$ ruby-parse -e 'foo while cond'
  (send nil :cond)
  (send nil :foo))
$ ruby-parse -e '(foo) while cond'
  (send nil :cond)
    (send nil :foo)))

(Parser also needs the (kwbegin) node type internally, and it is highly problematic to map it back to (begin).)

Backwards compatibility

Parser does not use semantic versioning. Parser versions are structured as x.y.z.t, where x.y.z indicates the most recent supported Ruby release (support for every Ruby release that is chronologically earlier is implied), and t is a monotonically increasing number.

The public API of Parser as well as the AST format (as listed in the documentation) are considered stable forever, although support for old Ruby versions may be removed at some point.

Sometimes it is necessary to modify the format of AST nodes that are already being emitted in a way that would break existing applications. To avoid such breakage, applications must opt-in to these modifications; without explicit opt-in, Parser will continue to emit the old AST node format. The most recent set of opt-ins is specified in the usage section of this README.

Compatibility with Ruby MRI

Unfortunately, Ruby MRI often changes syntax in patchlevel versions. This has happened, at least, for every release since 1.9; for example, commits c5013452 and 04bb9d6b were backported all the way from HEAD to 1.9. Moreover, there is no simple way to track these changes.

This policy makes it all but impossible to make Parser precisely compatible with the Ruby MRI parser. Indeed, at September 2014, it would be necessary to maintain and update ten different parsers together with their lexer quirks in order to be able to emulate any given released Ruby MRI version.

As a result, Parser chooses a different path: the parser/rubyXY parsers recognize the syntax of the latest minor version of Ruby MRI X.Y at the time of the gem release.

Compatibility with MacRuby and RubyMotion

Parser implements the MacRuby 0.12 and RubyMotion mid-2015 parsers precisely. However, the lexers of these have been forked off Ruby MRI and independently maintained for some time, and because of that, Parser may accept some code that these upstream implementations are unable to parse.

Known issues

Adding support for the following Ruby MRI features in Parser would needlessly complicate it, and as they all are very specific and rarely occurring corner cases, this is not done.

Parser has been extensively tested; in particular, it parses almost entire Rubygems corpus. For every issue, a breakdown of affected gems is offered.

Void value expressions

Ruby MRI prohibits so-called "void value expressions". For a description of what a void value expression is, see this gist and this Parser issue.

It is unknown whether any gems are affected by this issue.

Syntax check of block exits

Similar to "void value expression" checks Ruby MRI also checks for correct usage of break, next and redo, if it's used outside of a {break,next,redo}-able context Ruby returns a syntax error starting from 3.3.0. parser gem simply doesn't run this type of checks.

It is unknown whether any gems are affected by this issue.

Invalid characters inside comments and literals

Ruby MRI permits arbitrary non-7-bit byte sequences to appear in comments, as well as in string or symbol literals in form of escape sequences, regardless of source encoding. Parser requires all source code, including the expanded escape sequences, to consist of valid byte sequences in the source encoding that are convertible to UTF-8.

As of 2013-07-25, there are about 180 affected gems.

\u escape in 1.8 mode

Ruby MRI 1.8 permits to specify a bare \u escape sequence in a string; it treats it like u. Ruby MRI 1.9 and later treat \u as a prefix for Unicode escape sequence and do not allow it to appear bare. Parser follows 1.9+ behavior.

As of 2013-07-25, affected gems are: activerdf, activerdf_net7, fastreader, gkellog-reddy.


(This one is so obscure I couldn't even think of a saner name for this issue.) Pre-2.1 Ruby allows to specify a global variable named $-. Ruby 2.1 and later treat it as a syntax error. Parser follows 2.1 behavior.

No known code is affected by this issue.

EOF characters after embedded documents before 2.7

Code like "=begin\n""=end\0" is invalid for all versions of Ruby before 2.7. Ruby 2.7 and later parses it normally. Parser follows 2.7 behavior.

It is unknown whether any gems are affected by this issue.



The lexer testsuite is derived from ruby_parser.

The Bison parser rules are derived from Ruby MRI parse.y.


  1. Make sure you have Ragel ~> 6.7 installed
  2. Fork it
  3. Create your feature branch (git checkout -b my-new-feature)
  4. Commit your changes (git commit -am 'Add some feature')
  5. Push to the branch (git push origin my-new-feature)
  6. Create new Pull Request