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David Eisinger
2023-10-24 20:48:09 -04:00
parent 0438a6d828
commit f86f391e82
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content/elsewhere/write-you-a-parser-for-fun-and-win/index.md
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@@ -2,7 +2,6 @@
title: "Write You a Parser for Fun and Win"
date: 2013-11-26T00:00:00+00:00
draft: false
needs_review: true
canonical_url: https://www.viget.com/articles/write-you-a-parser-for-fun-and-win/
---
@@ -76,46 +75,49 @@ constructing parsers in the PEG (Parsing Expression Grammar) fashion."
Parslet turned out to be the perfect tool for the job. Here, for
example, is a basic parser for the above degree input:
class DegreeParser < Parslet::Parser
root :degree_groups
```ruby
class DegreeParser < Parslet::Parser
root :degree_groups
rule(:degree_groups) { degree_group.repeat(0, 1) >>
additional_degrees.repeat(0) }
rule(:degree_groups) { degree_group.repeat(0, 1) >>
additional_degrees.repeat(0) }
rule(:degree_group) { institution_name >>
(newline >> degree).repeat(1).as(:degrees_attributes) }
rule(:degree_group) { institution_name >>
(newline >> degree).repeat(1).as(:degrees_attributes) }
rule(:additional_degrees) { blank_line.repeat(2) >> degree_group }
rule(:additional_degrees) { blank_line.repeat(2) >> degree_group }
rule(:institution_name) { line.as(:institution_name) }
rule(:institution_name) { line.as(:institution_name) }
rule(:degree) { year.as(:year).maybe >>
semicolon >>
name >>
semicolon >>
field_of_study }
rule(:degree) { year.as(:year).maybe >>
semicolon >>
name >>
semicolon >>
field_of_study }
rule(:name) { segment.as(:name) }
rule(:field_of_study) { segment.as(:field_of_study) }
rule(:name) { segment.as(:name) }
rule(:year) { spaces >>
match("[0-9]").repeat(4, 4) >>
spaces }
rule(:field_of_study) { segment.as(:field_of_study) }
rule(:line) { spaces >>
match('[^ \r\n]').repeat(1) >>
match('[^\r\n]').repeat(0) }
rule(:year) { spaces >>
match("[0-9]").repeat(4, 4) >>
spaces }
rule(:segment) { spaces >>
match('[^ ;\r\n]').repeat(1) >>
match('[^;\r\n]').repeat(0) }
rule(:line) { spaces >>
match('[^ \r\n]').repeat(1) >>
match('[^\r\n]').repeat(0) }
rule(:blank_line) { spaces >> newline >> spaces }
rule(:newline) { str("\r").maybe >> str("\n") }
rule(:semicolon) { str(";") }
rule(:space) { str(" ") }
rule(:spaces) { space.repeat(0) }
end
rule(:segment) { spaces >>
match('[^ ;\r\n]').repeat(1) >>
match('[^;\r\n]').repeat(0) }
rule(:blank_line) { spaces >> newline >> spaces }
rule(:newline) { str("\r").maybe >> str("\n") }
rule(:semicolon) { str(";") }
rule(:space) { str(" ") }
rule(:spaces) { space.repeat(0) }
end
```
Let's take this line-by-line:
@@ -167,13 +169,15 @@ newline, etc.) are part of a parent class so that only the
resource-specific instructions would be included in this parser. Here's
what we get when we pass our degree info to this new parser:
[{:institution_name=>"Duke University"@0,
:degrees_attributes=>
[{:name=>" Ph.D."@17, :field_of_study=>" Biomedical Engineering"@24}]},
{:institution_name=>"University of North Carolina"@49,
:degrees_attributes=>
[{:year=>"2010"@78, :name=>" M.S."@83, :field_of_study=>" Biology"@89},
{:year=>"2007"@98, :name=>" B.S."@103, :field_of_study=>" Biology"@109}]}]
```ruby
[{:institution_name=>"Duke University"@0,
:degrees_attributes=>
[{:name=>" Ph.D."@17, :field_of_study=>" Biomedical Engineering"@24}]},
{:institution_name=>"University of North Carolina"@49,
:degrees_attributes=>
[{:year=>"2010"@78, :name=>" M.S."@83, :field_of_study=>" Biology"@89},
{:year=>"2007"@98, :name=>" B.S."@103, :field_of_study=>" Biology"@109}]}]
```
The values are Parslet nodes, and the `@XX` indicates where in the input
the rule was matched. With a little bit of string coercion, this output