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The following ABNF definition specifies the Brief Syntax of the SNOMED CT Expression Constraint Language. 
This ABNF syntax and the ANTLR syntax is maintained in the SNOMED Expression Constraint Language GitHub repository.

expressionConstraint = ws ( refinedExpressionConstraint / compoundExpressionConstraint /

simpleExpressionConstraint / "(" ws expressionConstraint ws ")" ) ws
simpleExpressionConstraint = [constraintOperator ws] eclFocusConcept
refinedExpressionConstraint = simpleExpressionConstraint ws ":" ws eclRefinement
compoundExpressionConstraint = conjunctionExpressionConstraint / disjunctionExpressionConstraint / exclusionExpressionConstraint
conjunctionExpressionConstraint = subExpressionConstraint 1*(ws conjunction ws subExpressionConstraint)
disjunctionExpressionConstraint = subExpressionConstraint 1*(ws disjunction ws subExpressionConstraint)
exclusionExpressionConstraint = subExpressionConstraint ws exclusion ws subExpressionConstraint
subExpressionConstraint = simpleExpressionConstraint / "(" ws expressionConstraint ws ")"
eclFocusConcept = [ memberOf ws ] (conceptReference / wildCard)
memberOf = "^"
conceptReference = conceptId [ws "|" ws term ws "|"]
conceptId = sctId
term = 1*nonwsNonPipe *( 1*SP 1*nonwsNonPipe )
wildCard = "*"
constraintOperator = childOf / descendantOrSelfOf / descendantOf / parentOf / ancestorOrSelfOf / ancestorOf
descendantOf = "<"
descendantOrSelfOf = "<<"
childOf = "<!"
ancestorOf = ">"
ancestorOrSelfOf = ">>"
parentOf = ">!"
conjunction =

 dottedExpressionConstraint / subExpressionConstraint ) ws
refinedExpressionConstraint = subExpressionConstraint ws ":" ws eclRefinement
compoundExpressionConstraint = conjunctionExpressionConstraint / disjunctionExpressionConstraint / exclusionExpressionConstraint
conjunctionExpressionConstraint = subExpressionConstraint 1*(ws conjunction ws subExpressionConstraint)
disjunctionExpressionConstraint = subExpressionConstraint 1*(ws disjunction ws subExpressionConstraint)
exclusionExpressionConstraint = subExpressionConstraint ws exclusion ws subExpressionConstraint
dottedExpressionConstraint = subExpressionConstraint 1*(ws dottedExpressionAttribute)
dottedExpressionAttribute = dot ws eclAttributeName
subExpressionConstraint= [constraintOperator ws] ( ( [memberOf ws] (eclFocusConcept / "(" ws expressionConstraint ws ")") *(ws memberFilterConstraint)) / (eclFocusConcept / "(" ws expressionConstraint ws ")") ) *(ws (descriptionFilterConstraint / conceptFilterConstraint)) [ws historySupplement]
eclFocusConcept = eclConceptReference / wildCard / altIdentifier
dot = "."
memberOf = "^" [ ws "[" ws (refsetFieldNameSet / wildCard) ws "]" ]
refsetFieldNameSet = refsetFieldName *(ws "," ws refsetFieldName)
refsetFieldName = 1*alpha
eclConceptReference = conceptId [ws "|" ws term ws "|"]
eclConceptReferenceSet = "(" ws eclConceptReference 1*(mws eclConceptReference) ws ")"
conceptId = sctId
term = 1*nonwsNonPipe *( 1*SP 1*nonwsNonPipe )
altIdentifier = (QM altIdentifierSchemeAlias "#" altIdentifierCodeWithinQuotes QM / altIdentifierSchemeAlias "#" altIdentifierCodeWithoutQuotes) [ws "|" ws term ws "|"]
altIdentifierSchemeAlias = alpha *(dash / alpha / integerValue)
altIdentifierCodeWithinQuotes = 1*anyNonEscapedChar
altIdentifierCodeWithoutQuotes = 1*(alpha / digit / dash / "." / "_")
wildCard = "*"
constraintOperator = childOf / childOrSelfOf / descendantOrSelfOf / descendantOf / parentOf / parentOrSelfOf / ancestorOrSelfOf / ancestorOf / top / bottom
descendantOf = "<"
descendantOrSelfOf = "<<"
childOf = "<!"
childOrSelfOf = "<<!"
ancestorOf = ">"
ancestorOrSelfOf = ">>"
parentOf = ">!"
parentOrSelfOf = ">>!"
top = "!!>"
bottom = "!!<"
conjunction = (("a"/"A") ("n"/"N") ("d"/"D") mws) / ","

disjunction = ("o"/"O") ("r"/"R") mws

exclusion = ("m"/"M") ("i"/"I") ("n"/"N") ("u"/"U") ("s"/"S") mws

eclRefinement = subRefinement ws [conjunctionRefinementSet / disjunctionRefinementSet]

conjunctionRefinementSet = 1*(ws conjunction ws subRefinement)

disjunctionRefinementSet = 1*(ws disjunction ws subRefinement)

subRefinement = eclAttributeSet / eclAttributeGroup / "(" ws eclRefinement ws ")"

eclAttributeSet = subAttributeSet ws [conjunctionAttributeSet / disjunctionAttributeSet]

conjunctionAttributeSet = 1*(ws conjunction ws subAttributeSet)

disjunctionAttributeSet = 1*(ws disjunction ws subAttributeSet)

subAttributeSet = eclAttribute / "(" ws eclAttributeSet ws ")"

eclAttributeGroup = ["[" cardinality

ws

 "]" ws] "{" ws eclAttributeSet ws "}"

eclAttribute = ["[" cardinality "]" ws] [reverseFlag ws]

[attributeOperator ws]

 eclAttributeName ws (expressionComparisonOperator ws

expressionConstraintValue

 subExpressionConstraint / numericComparisonOperator ws "#" numericValue / stringComparisonOperator

ws QM stringValue QM)
cardinality = minValue to maxValue
minValue = nonNegativeIntegerValue
to =

 ws (typedSearchTerm / typedSearchTermSet) / booleanComparisonOperator ws booleanValue)
cardinality = minValue to maxValue
minValue = nonNegativeIntegerValue
to = ".."

maxValue = nonNegativeIntegerValue / many

many = "*"

reverseFlag = "R

"
attributeOperator = descendantOrSelfOf / descendantOf
eclAttributeName = conceptReference / wildCard
expressionConstraintValue = simpleExpressionConstraint / "(" ws (refinedExpressionConstraint / compoundExpressionConstraint) ws ")"
expressionComparisonOperator = "=" / "!="
numericComparisonOperator = "=" / "!=" / "<=" / "<" / ">=" / ">"
stringComparisonOperator = "=" / "!="
numericValue = decimalValue / integerValue
stringValue = 1*(anyNonEscapedChar / escapedChar)
integerValue = ( ["-"/"+"] digitNonZero *digit ) / zero
decimalValue = integerValue "." 1*digit
nonNegativeIntegerValue = (digitNonZero *digit ) / zero
sctId = digitNonZero 5*17( digit )
ws = *( SP / HTAB / CR / LF / comment ) ; optional white space
mws = 1*( SP / HTAB / CR / LF / comment ) ; mandatory white space
comment = "/*" *(nonStarChar / starWithNonFSlash) "*/"
nonStarChar = SP / HTAB / CR / LF / %x21-29 / %x2B-7E /UTF8-2 / UTF8-3 / UTF8-4
starWithNonFSlash = %x2A nonFSlash
nonFSlash = SP / HTAB / CR / LF / %x21-2E / %x30-7E /UTF8-2 / UTF8-3 / UTF8-4
SP = %x20 ; space
HTAB = %x09 ; tab
CR = %x0D ; carriage return
LF = %x0A ; line feed
QM = %x22 ; quotation mark
BS = %x5C ; back slash
digit = %x30-39
zero = %x30
digitNonZero = %x31-39
nonwsNonPipe = %x21-7B / %x7D-7E / UTF8-2 / UTF8-3 / UTF8-4
anyNonEscapedChar = SP / HTAB / CR / LF / %x20-21 / %x23-5B / %x5D-7E / UTF8-2 / UTF8-3 / UTF8-4
escapedChar = BS QM / BS BS
UTF8-2 = %xC2-DF UTF8-tail
UTF8-3 = %xE0 %xA0-BF UTF8-tail / %xE1-EC 2( UTF8-tail ) / %xED %x80-9F UTF8-tail / %xEE-EF 2( UTF8-tail )
UTF8-4 = %xF0 %x90-BF 2( UTF8-tail ) / %xF1-F3 3( UTF8-tail ) / %xF4 %x80-8F 2( UTF8-tail )
UTF8-tail = %x80-BF

...

"
eclAttributeName = subExpressionConstraint
expressionComparisonOperator = "=" / "!="
numericComparisonOperator = "=" / "!=" / "<=" / "<" / ">=" / ">"
timeComparisonOperator = "=" / "!=" / "<=" / "<" / ">=" / ">"
stringComparisonOperator = "=" / "!="
booleanComparisonOperator = "=" / "!="
idComparisonOperator = "=" / "!="
descriptionFilterConstraint = "{{" ws [ "d" / "D" ] ws descriptionFilter *(ws "," ws descriptionFilter) ws "}}"
descriptionFilter = termFilter / languageFilter / typeFilter / dialectFilter / moduleFilter / effectiveTimeFilter / activeFilter / descriptionIdFilter
descriptionIdFilter = descriptionIdKeyword ws idComparisonOperator ws (descriptionId / descriptionIdSet)
descriptionIdKeyword = ("i"/"I") ("d"/"D")
descriptionId = sctId
descriptionIdSet = "(" ws descriptionId *(mws descriptionId) ws ")"
termFilter = termKeyword ws stringComparisonOperator ws (typedSearchTerm / typedSearchTermSet)
termKeyword = ("t"/"T") ("e"/"E") ("r"/"R") ("m"/"M")
typedSearchTerm = ( [ matchKeyword ws ":" ws ] matchSearchTermSet ) / ( wild ws ":" ws wildSearchTermSet )
typedSearchTermSet = "(" ws typedSearchTerm *(mws typedSearchTerm) ws ")"
wild = ("w"/"W") ("i"/"I") ("l"/"L") ("d"/"D")
matchKeyword = ("m"/"M") ("a"/"A") ("t"/"T") ("c"/"C") ("h"/"H")
matchSearchTerm = 1*(nonwsNonEscapedChar / escapedChar)
matchSearchTermSet = QM ws matchSearchTerm *(mws matchSearchTerm) ws QM
wildSearchTerm = 1*(anyNonEscapedChar / escapedWildChar)
wildSearchTermSet = QM wildSearchTerm QM
languageFilter = language ws booleanComparisonOperator ws (languageCode / languageCodeSet)
language = ("l"/"L") ("a"/"A") ("n"/"N") ("g"/"G") ("u"/"U") ("a"/"A") ("g"/"G") ("e"/"E")
languageCode = 2alpha
languageCodeSet = "(" ws languageCode *(mws languageCode) ws ")"
typeFilter = typeIdFilter / typeTokenFilter
typeIdFilter = typeId ws booleanComparisonOperator ws (subExpressionConstraint / eclConceptReferenceSet)
typeId = ("t"/"T") ("y"/"Y") ("p"/"P") ("e"/"E") ("i"/"I") ("d"/"D")
typeTokenFilter = type ws booleanComparisonOperator ws (typeToken / typeTokenSet)
type = ("t"/"T") ("y"/"Y") ("p"/"P") ("e"/"E")
typeToken = synonym / fullySpecifiedName / definition
typeTokenSet = "(" ws typeToken *(mws typeToken) ws ")"
synonym = ("s"/"S") ("y"/"Y") ("n"/"N")
fullySpecifiedName = ("f"/"F") ("s"/"S") ("n"/"N")
definition = ("d"/"D") ("e"/"E") ("f"/"F")
dialectFilter = (dialectIdFilter / dialectAliasFilter) [ ws acceptabilitySet ]
dialectIdFilter = dialectId ws booleanComparisonOperator ws (subExpressionConstraint / dialectIdSet)
dialectId = ("d"/"D") ("i"/"I") ("a"/"A") ("l"/"L") ("e"/"E") ("c"/"C") ("t"/"T") ("i"/"I") ("d"/"D")
dialectAliasFilter = dialect ws booleanComparisonOperator ws (dialectAlias / dialectAliasSet)
dialect = ("d"/"D") ("i"/"I") ("a"/"A") ("l"/"L") ("e"/"E") ("c"/"C") ("t"/"T")
dialectAlias = alpha *( dash / alpha / integerValue)
dialectAliasSet = "(" ws dialectAlias [ws acceptabilitySet] *(mws dialectAlias [ws acceptabilitySet] ) ws ")"
dialectIdSet = "(" ws eclConceptReference [ws acceptabilitySet] *(mws eclConceptReference [ws acceptabilitySet] ) ws ")"
acceptabilitySet = acceptabilityConceptReferenceSet / acceptabilityTokenSet
acceptabilityConceptReferenceSet = "(" ws eclConceptReference *(mws eclConceptReference) ws ")"
acceptabilityTokenSet = "(" ws acceptabilityToken *(mws acceptabilityToken) ws ")"
acceptabilityToken = acceptable / preferred
acceptable = ("a"/"A") ("c"/"C") ("c"/"C") ("e"/"E") ("p"/"P") ("t"/"T")
preferred = ("p"/"P") ("r"/"R") ("e"/"E") ("f"/"F") ("e"/"E") ("r"/"R")
conceptFilterConstraint = "{{" ws ("c" / "C") ws conceptFilter *(ws "," ws conceptFilter) ws "}}"
conceptFilter = definitionStatusFilter / moduleFilter / effectiveTimeFilter / activeFilter
definitionStatusFilter = definitionStatusIdFilter / definitionStatusTokenFilter
definitionStatusIdFilter = definitionStatusIdKeyword ws booleanComparisonOperator ws (subExpressionConstraint / eclConceptReferenceSet)
definitionStatusIdKeyword = ("d"/"D") ("e"/"E") ("f"/"F") ("i"/"I") ("n"/"N") ("i"/"I") ("t"/"T") ("i"/"I") ("o"/"O") ("n"/"N") ("s"/"S") ("t"/"T") ("a"/"A") ("t"/"T") ("u"/"U") ("s"/"S") ("i"/"I") ("d"/"D")
definitionStatusTokenFilter = definitionStatusKeyword ws booleanComparisonOperator ws (definitionStatusToken / definitionStatusTokenSet)
definitionStatusKeyword = ("d"/"D") ("e"/"E") ("f"/"F") ("i"/"I") ("n"/"N") ("i"/"I") ("t"/"T") ("i"/"I") ("o"/"O") ("n"/"N") ("s"/"S") ("t"/"T") ("a"/"A") ("t"/"T") ("u"/"U") ("s"/"S")
definitionStatusToken = primitiveToken / definedToken
definitionStatusTokenSet = "(" ws definitionStatusToken *(mws definitionStatusToken) ws ")"
primitiveToken = ("p"/"P") ("r"/"R") ("i"/"I") ("m"/"M") ("i"/"I") ("t"/"T") ("i"/"I") ("v"/"V") ("e"/"E")
definedToken = ("d"/"D") ("e"/"E") ("f"/"F") ("i"/"I") ("n"/"N") ("e"/"E") ("d"/"D")
moduleFilter = moduleIdKeyword ws booleanComparisonOperator ws (subExpressionConstraint / eclConceptReferenceSet)
moduleIdKeyword = ("m"/"M") ("o"/"O") ("d"/"D") ("u"/"U") ("l"/"L") ("e"/"E") ("i"/"I") ("d"/"D")
effectiveTimeFilter = effectiveTimeKeyword ws timeComparisonOperator ws ( timeValue / timeValueSet )
effectiveTimeKeyword = ("e"/"E") ("f"/"F") ("f"/"F") ("e"/"E") ("c"/"C") ("t"/"T") ("i"/"I") ("v"/"V") ("e"/"E") ("t"/"T") ("i"/"I") ("m"/"M") ("e"/"E")
timeValue = QM [ year month day ] QM
timeValueSet = "(" ws timeValue *(mws timeValue) ws ")"
year = digitNonZero digit digit digit
month = "01" / "02" / "03" / "04" / "05" / "06" / "07" / "08" / "09" / "10" / "11" / "12"
day = "01" / "02" / "03" / "04" / "05" / "06" / "07" / "08" / "09" / "10" / "11" / "12" / "13" / "14" / "15" / "16" / "17" / "18" / "19" / "20" / "21" / "22" / "23" / "24" / "25" / "26" / "27" / "28" / "29" / "30" / "31"
activeFilter = activeKeyword ws booleanComparisonOperator ws activeValue
activeKeyword = ("a"/"A") ("c"/"C") ("t"/"T") ("i"/"I") ("v"/"V") ("e"/"E")
activeValue = activeTrueValue / activeFalseValue
activeTrueValue = "1" / "true"
activeFalseValue = "0" / "false"
memberFilterConstraint = "{{" ws ("m" / "M") ws memberFilter *(ws "," ws memberFilter) ws "}}"
memberFilter = moduleFilter / effectiveTimeFilter / activeFilter / memberFieldFilter
memberFieldFilter = refsetFieldName ws (expressionComparisonOperator ws subExpressionConstraint / numericComparisonOperator ws "#" numericValue / stringComparisonOperator ws (typedSearchTerm / typedSearchTermSet) / booleanComparisonOperator ws booleanValue / ws timeComparisonOperator ws (timeValue / timeValueSet) )
historySupplement = "{{" ws "+" ws historyKeyword [ historyProfileSuffix / ws historySubset ] ws "}}"
historyKeyword = ("h"/"H") ("i"/"I") ("s"/"S") ("t"/"T") ("o"/"O") ("r"/"R") ("y"/"Y")
historyProfileSuffix = historyMinimumSuffix / historyModerateSuffix / historyMaximumSuffix
historyMinimumSuffix = ("-"/"_") ("m"/"M") ("i"/"I") ("n"/"N")
historyModerateSuffix = ("-"/"_") ("m"/"M") ("o"/"O") ("d"/"D")
historyMaximumSuffix = ("-"/"_") ("m"/"M") ("a"/"A") ("x"/"X")
historySubset = "(" ws expressionConstraint ws ")"
numericValue = ["-"/"+"] (decimalValue / integerValue)
stringValue = 1*(anyNonEscapedChar / escapedChar)
integerValue = digitNonZero *digit / zero
decimalValue = integerValue "." 1*digit
booleanValue = true / false
true = ("t"/"T") ("r"/"R") ("u"/"U") ("e"/"E")
false = ("f"/"F") ("a"/"A") ("l"/"L") ("s"/"S") ("e"/"E")
nonNegativeIntegerValue = (digitNonZero *digit ) / zero
sctId = digitNonZero 5*17( digit )
ws = *( SP / HTAB / CR / LF / comment ) ; optional white space
mws = 1*( SP / HTAB / CR / LF / comment ) ; mandatory white space
comment = "/*" *(nonStarChar / starWithNonFSlash) "*/"
nonStarChar = SP / HTAB / CR / LF / %x21-29 / %x2B-7E /UTF8-2 / UTF8-3 / UTF8-4
starWithNonFSlash = %x2A nonFSlash
nonFSlash = SP / HTAB / CR / LF / %x21-2E / %x30-7E /UTF8-2 / UTF8-3 / UTF8-4
SP = %x20 ; space
HTAB = %x09 ; tab
CR = %x0D ; carriage return
LF = %x0A ; line feed
QM = %x22 ; quotation mark
BS = %x5C ; back slash
star = %x2A ; asterisk
digit = %x30-39
zero = %x30
digitNonZero = %x31-39
nonwsNonPipe = %x21-7B / %x7D-7E / UTF8-2 / UTF8-3 / UTF8-4
anyNonEscapedChar = SP / HTAB / CR / LF / %x20-21 / %x23-5B / %x5D-7E / UTF8-2 / UTF8-3 / UTF8-4
escapedChar = BS QM / BS BS
escapedWildChar = BS QM / BS BS / BS star
nonwsNonEscapedChar = %x21 / %x23-5B / %x5D-7E / UTF8-2 / UTF8-3 / UTF8-4
alpha = %x41-5A / %x61-7A
dash = %x2D
UTF8-2 = %xC2-DF UTF8-tail
UTF8-3 = %xE0 %xA0-BF UTF8-tail / %xE1-EC 2( UTF8-tail ) / %xED %x80-9F UTF8-tail / %xEE-EF 2( UTF8-tail )
UTF8-4 = %xF0 %x90-BF 2( UTF8-tail ) / %xF1-F3 3( UTF8-tail ) / %xF4 %x80-8F 2( UTF8-tail )
UTF8-tail = %x80-BF