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Structural pattern matching against S-Expressions.

SexpPath allows you to define patterns that can be matched against S-Expressions. It draws inspiration from Regular Expressions, XPath, and CSS Selectors.

I’m still figuring out how this should work so either fork this, or send me some feedback.


SexpPath is distributed as a ruby gem:

gem install sexp_path


In ruby you’re most likely to come across S-Expressions when dealing with Ruby’s representation of the abstract syntax tree. An S-Expression is just a set of nested lists. The SexpProcessor library displays them like this:

s(:a, :b, 

Where that means that there is a list containing ‘:a`, `:b`, and then another list which contains `:c`. We will refer to `:a`,`:b`, and `:c` as atoms, while `s( something )` is an S-Expression or Sexp.

General Syntax

SexpPath is an internal ruby DSL, which means a SexpPath query is valid ruby code.

SexpPath queries are built with the SexpQueryBuilder through the Q? convenience method:

Q?{ s(:a, :b, :c)}    # Matches s(:a, :b, :c)

This will match the S-Expression ‘s(:a, :b, :c)`. If you want to match something more complicated, you will probably want to employ one of the many matchers built into SexpPath.

Wild Card

Matches anything.

_  => s(), :a, or s(:cat)

Matches any atom (or symbol).

atom => :a, :b, or :cat

Matches any atom that matches the given string or regular expression.

m('cat') => :cat
m(/rat/) => :rat, :brat, or :rate
m(/^test_/) => :test_sexp_path

Matches any S-Expression that includes the sub expression.

include(:a) => s(:a), s(:a, :b), or s(:cat, :a, :b)
include( s(:cat) ) => s(:pet, s(:cat))

Matches any S-Expression that has the sub expression as a child.

child( s(:a) ) => s(:b, s(:a)) or even s(s(s(s(s( s(:a))))))

Matches any S-Expression that has the second expression as a sibling.

s(:a) >> s(:c) => s( s(:a), s(:b), s(:c) )

The sexp type is considered to be the first atom. This matches any expression that has the given type.

type(:a) => s(:a), s(:a, :b), or s(:a, :b, s(:c))

Matches any sub expression

any( s(:a), s(:b) ) => s(:a) or s(:b)
any( s(:a), s(atom, :b) ) => s(:a), s(:a, :b), or s(:cat, :b)

Matches anything that satisfies all the sub expressions

all( s(:a, atom), s(atom, :b) ) => s(:a,:b)

Negates a matcher

-s(:a) => s(:a,:b), s(:b), but not s(:a)
s(is_not :a) => s(:b), s(:c), but not s(:a) or s(:a, :b)

Matches the remainder of an s-expression.

s(:a, ___)  => s(:a), s(:a, :b, :c), but not s(:b)


You may use any SexpPath to search an S-Expression. SexpPath defines the ‘/` operator as search, so to search `s( s(:a) )` for `s(:a)` you may just do:

s( s(:a) ) / Q?{ s(:a) }

This will return a collection with just one result which is ‘s(:a)`. You could also do something more interesting:

s( s(:a), s(:b) ) / Q?{ s(atom) }

This will return two matches which are ‘s(:a)` and `s(:b)`. You can also chain searches, so this works just fine as well:

sexp = s(:class, :Calculator,
  s(:defn, :add),
  s(:defn, :sub)

sexp / Q?{ s(:class, atom, _) } / Q?{ s(:defn, _) }

In this case you would get back ‘s(:defn, :add)` and `s(:defn, :sub)`.


It is useful to also capture results from your queries. So using the same Sexp from above we could modify our query to actually capture some names. Capturing is done by using ‘%` operator followed by the name you would like the value to be captured as.

sexp / Q?{ s(:class, atom % 'class_name', _) } / Q?{ s(:defn, _ % 'method_name') }

The results will now capture ‘:Calculator` in `class_name`, and then `:add` and `:sub` in `method_name`.


Here is an example of using SexpPath to grab all the classes and their methods from a file:

require 'sexp_path'
require 'ruby_parser' # `gem install ruby_parser`

path = ARGV.shift
code =
sexp =, path)

class_query = Q?{ s(:class, atom % 'class_name', _, ___) }
method_query = Q?{ s(:defn, atom % 'method_name', ___) }

results = sexp / class_query / method_query

puts path
puts "-" * 80

results.each do |sexp_result|
  class_name = sexp_result['class_name']
  method_name = sexp_result['method_name']
  puts "#{class_name}##{method_name}"

Neat huh? Check the ‘examples` folder for some more little apps.

Project Information

Hop in and fork it or add some issues over at GitHub:

Ideas for Hacking on SexpPath:

* More examples
* Add new matchers
* Convenience matchers, for instance canned matchers for matching ruby classes, methods, etc

I’d love to see what people do with this library, let me know if you find it useful.

Adam Sanderson,


Structural pattern matching for S-Expressions






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