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Ragel is a finite-state machine compiler and a parser generator. Initially Ragel supported output for C, C++ and Assembly source code,[1] was expanded to support several other languages including Objective C, D, Go, Ruby, and Java.[2] Additional language support is also in development.[3] It supports the generation of table or control flow driven state machines from regular expressions[4] and/or state charts and can also build lexical analysers via the longest-match method. Ragel specifically targets text parsing and input validation.[5]

Developer(s)Adrian Thurston[6]
Stable release
6.10 / March 24, 2017; 3 years ago (2017-03-24)
Preview release / May 14, 2019; 20 months ago (2019-05-14)
Repository Edit this at Wikidata
Written inC++
Operating systemUnix-like, Windows
TypeState machine compiler
License"Ragel 6 remains under GPL v2 [generated code] covered by the MIT (or GPL v2)".[7]
Ragel 7: MIT License


Ragel supports the generation of table or control flow driven state machines from regular expressions and/or state charts and can also build lexical analysers via the longest-match method. A unique feature of Ragel is that user actions can be associated with arbitrary state machine transitions using operators that are integrated into the regular expressions. Ragel also supports visualization of the generated machine via graphviz.

Visualisierung Ragel.png

The graph represents a state-machine that takes user input as a series of bytes representing ASCII characters and control codes. 48..57 is equivalent to the regular expression [0-9] (i.e. any digit), so only sequences beginning with a digit can be recognised. If 10 (line feed) is encountered, we're done. 46 is the decimal point ('.'), 43 and 45 are positive and negative signs ('+', '-') and 69/101 is uppercase/lowercase 'e' (to indicate a number in scientific format). As such it will recognize the following properly:


but not:



Ragel's input is a regular expression only in the sense that it describes a regular language; it is usually not written in a concise regular expression, but written out into multiple parts like in Extended Backus–Naur form. For example, instead of supporting POSIX character classes in regex syntax, Ragel implements them as built-in production rules. As with usual parser generators, Ragel allows for handling code for productions to be written with the syntax.[8] The code yielding the above example from the official website is:

action dgt      { printf("DGT: %c\n", fc); }
action dec      { printf("DEC: .\n"); }
action exp      { printf("EXP: %c\n", fc); }
action exp_sign { printf("SGN: %c\n", fc); }
action number   { /*NUMBER*/ }

# A floating-point number literal.
number = (
    [0-9]+ $dgt ( '.' @dec [0-9]+ $dgt )?
    ( [eE] ( [+\-] $exp_sign )? [0-9]+ $exp )?
) %number;

main := ( number '\n' )*;

See also[edit]


  1. ^ Adrian D. Thurston. "Parsing Computer Languages with an Automaton Compiled from a Single Regular Expression. Archived 2012-09-07 at the Wayback Machine" In: 11th International Conference on Implementation and Application of Automata (CIAA 2006), Lecture Notes in Computer Science, volume 4094, p. 285-286, Taipei, Taiwan, August 2006.
  2. ^ "Ragel User Guide" (PDF). March 2017.
  3. ^ "Additional Target Languages Return to Ragel 7". 18 May 2018.
  4. ^ Liqun Chen, Chris J. Mitchell, Andrew Martin (2009) Trusted Computing: Second International Conference, Trust 2009 Oxford, UK, April 6–8, 2009, Proceedings. p. 111
  5. ^ Omar Badreddin (2010) "Umple: a model-oriented programming language." Software Engineering, 2010 ACM/IEEE 32nd International Conference on. Vol. 2. IEEE, 2010.
  6. ^ Dr. Adrian D. Thurston at complang.org Last changed: Jul 14, 2013
  7. ^ "Ragel State Machine Compiler". www.colm.net. Retrieved 2019-11-19.
  8. ^ "Ragel User Guide" (PDF). March 2017.

External links[edit]