An ABNF Primer
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An introduction to W3C's ABNF grammar syntax.
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An ABNF Primer
- 1. An ABNF PrimerConcepts, structure, syntax
- 6. Defines both the XML form and the Augmented BNF (ABNF) form
- 8. Supported by a complete development environment
- 9. NuGram IDECopyright © 2010 Nu Echo Inc.
- 10. Structure of an ABNF Grammar Headers #ABNF 1.0 ISO-8859-1; language en-US; mode voice; root$yesOrNo; private$yesOrNo = yes {out.value = 'yes'} | no {out.value = 'no'} ; Rules Copyright © 2010 Nu Echo Inc.
- 12. // to end of line
- 13. /* ... */
- 15. /** ... */
- 16. @example to document sample sentences
- 17. Can appear before rule definitions, root header, and language header.Copyright © 2010 Nu Echo Inc.
- 18. Grammar headers Copyright © 2010 Nu Echo Inc.
- 20. Mandatory
- 22. Can be preceded by byte order mask (BOM)Ex: #ABNF 1.0; #ABNF 1.0 UTF-8; Copyright © 2010 Nu Echo Inc.
- 24. Identifies the language of the document
- 25. Required for voice grammars
- 26. Languages codes defined by RFC 3066Ex: language en; language fr-CA; Copyright © 2010 Nu Echo Inc.
- 28. Indicates the type of input
- 29. Optional header
- 30. 'dtmf' or 'voice' (default)Ex: mode voice; mode dtmf; Copyright © 2010 Nu Echo Inc.
- 32. Defines the grammar's top-level rule
- 33. Optional
- 34. Root rule can be either public or privateEx: root$rootRule; Copyright © 2010 Nu Echo Inc.
- 36. Declares content type for the semantic tags (actions)
- 37. Value is a URI
- 38. Recognition-engine specificEx: tag-format <semantics/1.0>; (SISR) tag-format <Nuance>; (Nuance) tag-format <swi-semantics/1.0>; (Nuance) tag-format <semantics-ms/1.0>; (Microsoft) tag-format <semantics/1.0.2006>; (LumenVox) Copyright © 2010 Nu Echo Inc.
- 40. Specifies a pronunciation lexicon to use (a URI)
- 41. One or more occurrences
- 42. Format is engine-specificEx: lexicon <../lex/names.pls>; Copyright © 2010 Nu Echo Inc.
- 44. Base URI for all relative URIs in the document
- 45. Optional
- 46. Has precedence over the meta headerEx: base <http://localhost:8080/>; Copyright © 2010 Nu Echo Inc.
- 48. Metadata attached to the grammar
- 49. Properties taking precedence over HTTP headers
- 50. One or more occurrencesEx: meta“Author”is“J. Doe”; http-equiv“Expires”is“0”; Copyright © 2010 Nu Echo Inc.
- 52. base header
- 54. metadata from protocol interaction (ex. HTTP header)
- 55. base URI of the current documentEx: meta“base”is“http://example.com/grammars/date.abnf”; meta“base”is“http://example.com/grammars/”; base “http://example.com/grammars/date.abnf”; Copyright © 2010 Nu Echo Inc.
- 57. OSR / Nuance 9
- 58. Full conformance to RFC 2396 (now obsoleted by RFC 3986)
- 59. Loquendo ASR
- 60. Base URI must not contain the document’s name
- 61. Best practice: always end the URI with a “/”Copyright © 2010 Nu Echo Inc.
- 62. Grammar rules Copyright © 2010 Nu Echo Inc.
- 64. Rule names are unique in a grammar
- 65. Rules can be either public or private
- 66. A public rule is visible outside the scope of the grammar document
- 67. A rule is private by default
- 68. An expansion describes a set of word sequences (sentences)Copyright © 2010 Nu Echo Inc.
- 72. But:
- 73. Cannot be NULL, VOID, GARBAGE
- 74. Must not contain '.', ':', '-'
- 75. Rule names begin with '$'
- 76. Ex: $number $names $digitsCopyright © 2010 Nu Echo Inc.
- 77. Basic Expansions Copyright © 2010 Nu Echo Inc.
- 79. A string that the recognizer can convert to a phonetic representation
- 80. Typically a word in the specified language
- 81. Words are delimited by whitespaces, and$, <, >, (, ), /, |, [, ], {, }, !
- 82. Can be enclosed in “”Copyright © 2010 Nu Echo Inc.
- 84. Examples:
- 85. hello
- 86. Montréal
- 87. trente-deux
- 88. “San Francisco”Copyright © 2010 Nu Echo Inc.
- 90. Tips:
- 91. Avoid acronyms, like “IBM” (unless word is in the dictionary)Separate each letter with a “.” or “_”.
- 92. Good: “I.B.M.”, “I triple E”
- 95. Replace “Dr.” with “Doctor”, “St.” with “street"
- 96. Use spelled forms for numbers
- 97. Use “One hundred” instead of “100”Copyright © 2010 Nu Echo Inc.
- 100. Each expansion must match in turnEx: I would like to $day $month $year Copyright © 2010 Nu Echo Inc.
- 102. To match one of a number of choices
- 103. Expansions separated by '|'Ex: one | two | three | four $yes | $no{answer = 'no'} Copyright © 2010 Nu Echo Inc.
- 105. Parentheses group expansions (as do square brackets)
- 107. Useful to ensure correct precedence in parsing a group of expansionsEx: no (it's | it is) not (oh | zero) { out = '0'} Copyright © 2010 Nu Echo Inc.
- 109. References to named expansions
- 110. Three types:
- 111. Local references
- 112. External references
- 113. Special rule namesCopyright © 2010 Nu Echo Inc.
- 115. References to rules declared in the same documentEx: $date $civicNumber $digit Copyright © 2010 Nu Echo Inc.
- 117. Implicit
- 118. Uses root rule
- 123. $NULL
- 125. equivalent to ()
- 126. $VOID
- 127. matches nothing
- 128. $GARBAGE
- 129. matches anything
- 130. implementation-specific behaviorCopyright © 2010 Nu Echo Inc.
- 131. Advanced expansions Copyright © 2010 Nu Echo Inc.
- 133. Force the repetition of an expansion
- 134. Bounded: <n-m> or <n>
- 135. Unbounded: <n->Ex: $digit <7> | $digit <10> $topping <1-> Copyright © 2010 Nu Echo Inc.
- 137. Special case: [ E ]
- 138. Equivalent to E <0-1>
- 139. Or to ( E | $NULL )Ex: $hesitation = [euh] ok [euh]; $date = $month [$year]; Copyright © 2010 Nu Echo Inc.
- 141. The probability of the repetition can be specified
- 142. Syntax: <n-m /prob/>
- 143. prob is value between 0.0 and 1.0Ex: $digit <2-4 /0.8/> $topping <1- /0.785/> Copyright © 2010 Nu Echo Inc.
- 145. Multiplying factor attached to a choice
- 146. Positively biased when weight > 1.0
- 147. Negatively biased when weight < 1.0
- 148. Default weight is 1.0
- 149. Syntax: /n./ or /n.n/ or /.n/ or /n/Ex: (/1.7/ New York | /0.4/ Newark) Copyright © 2010 Nu Echo Inc.
- 151. Overrides the default language
- 152. Applies to all words of the target expansion
- 153. Syntax: E!codeEx: yes | oui!fr-CA (Michel Tremblay|André Roy)!fr-CA Copyright © 2010 Nu Echo Inc.
- 155. To attach “meaning” to a sentence
- 156. Arbritrary strings enclosed in {}
- 157. Content depends on value of the tag-format headerEx: (two | second) {out.day = 1} $number{val = number.val} Copyright © 2010 Nu Echo Inc.
- 159. Use {!{ ... }!} when tag must contain '}'.
- 160. Can appear as grammar header
- 161. must be followed by ';‘
- 162. Ideal for defining global functions or data structuresCopyright © 2010 Nu Echo Inc.
- 163. Precedence Copyright © 2010 Nu Echo Inc.
Editor's Notes
- Note that Nuance GSL grammars support mixed-mode grammars. This is not possible with SRGS grammars.
- Note that in GSL grammars, if no rule is declared public, all rules are assumed public.
- Note that some engines will accept numbers (e.g., “100”). This, however, is a bad idea to rely on this because we don’t know how it will be rendered.
- Note that square brackets also group expansions
- Note that some engines have limits on the number of repeats. For instance, Loquendo has a maximum of 30.
- Note that the form “(E | $NULL)” is preferable when computing grammar weights based on data since we can have a weight computed for both branches.
- Note that this may not be supported by all engines. Even if it is, it should be tested carefully. For instance, this does not work well with OSR 3.0 (don’t know about Nuance 9).