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Gelatin is a parser generator for converting text to a structured format such as XML, JSON or YAML.

Do you need commercial support?

Gelatin is supported by Procedure 8. Get in touch if you need anything!

Converting Text to XML, JSON, or YAML

Gelatin is a combined lexer, parser, and output generator. Gelatin defines a simple language for converting text into a structured formats.


Suppose you want to convert the following text file to XML:

Name: John, Lastname: Doe
Office: 1st Ave
Birth date: 1978-01-01

Name: Jane, Lastname: Foo
Office: 2nd Ave
Birth date: 1970-01-01

The following Gelatin syntax does the job:

# Define commonly used data types. This is optional, but
# makes your life a litte easier by allowing to reuse regular
# expressions in the grammar.
define nl /[\r\n]/
define ws /\s+/
define fieldname /[\w ]+/
define value /[^\r\n,]+/
define field_end /[\r\n,] */

grammar user:
    match 'Name:' ws value field_end:
        out.add_attribute('.', 'firstname', '$2')
    match 'Lastname:' ws value field_end:
        out.add_attribute('.', 'lastname',  '$2')
    match fieldname ':' ws value field_end:
        out.add('$0', '$3')
    match nl:

# The grammar named "input" is the entry point for the converter.
grammar input:
    match 'User' nl '----' nl:'user')


  • "grammar input:" is the entry point for the converter.
  • "match" statements in each grammar are executed sequentially. If a match is found, the indented statements in the match block are executed. After reaching the end of a match block, the grammar restarts at the top of the grammar block.
  • If the end of a grammar is reached before the end of the input document was reached, an error is raised.
  • "out.add('$0', '$3')" creates a node in the XML (or JSON, or YAML) if it does not yet exist. The name of the node is the value of the first matched field (the fieldname, in this case). The data of the node is the value of the fourth matched field.
  • "'user')" creates a "user" node in the output and selects it such that all following "add" statements generate output relative to the "user" node. Gelatin leaves the user node upon reaching the out.leave() statement.
  • "user()" calls the grammar named "user".

This produces the following output:

  <user lastname="Doe" firstname="John">
    <office>1st Ave</office>
  <user lastname="Foo" firstname="Jane">
    <office>2nd Ave</office>

Method 1: Starting the transformation using the CLI tool

The following command converts the input to XML:

gel -s mysyntax.gel input.txt

The same for JSON or YAML:

gel -s mysyntax.gel -f json input.txt
gel -s mysyntax.gel -f yaml input.txt

Method 2: Starting the transformation using Gelatin as a Python Module

Gelatin also provides a Python API for transforming the text:

from Gelatin.util import compile, generate

# Parse your .gel file.
syntax = compile('syntax.gel')

# Convert your input file to XML, YAML, and JSON.
print(generate(syntax, 'input.txt'))
print(generate(syntax, 'input.txt', format='yaml'))
print(generate(syntax, 'input.txt', format='json'))


For full documentation please refer to