The following ABNF definition specifies the Brief Syntax of the SNOMED CT Expression Constraint Language.

 expressionConstraint = ws ( refinedExpressionConstraint / compoundExpressionConstraint / simpleExpressionConstraint / "(" ws expressionConstraint ws ")" ) ws
 simpleExpressionConstraint = [constraintOperator ws] focusConcept
 refinedExpressionConstraint = simpleExpressionConstraint ws ":" ws refinement
 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 ")"
 focusConcept = [ 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 = (("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 
 refinement = subRefinement ws [conjunctionRefinementSet / disjunctionRefinementSet]
 conjunctionRefinementSet = 1*(ws conjunction ws subRefinement)
 disjunctionRefinementSet = 1*(ws disjunction ws subRefinement)
 subRefinement = attributeSet / attributeGroup / "(" ws refinement ws ")"
 attributeSet = subAttributeSet ws [conjunctionAttributeSet / disjunctionAttributeSet]
 conjunctionAttributeSet = 1*(ws conjunction ws subAttributeSet)
 disjunctionAttributeSet = 1*(ws disjunction ws subAttributeSet)
 subAttributeSet = attribute / "(" ws attributeSet ws ")"
 attributeGroup = [cardinality ws] "{" ws attributeSet ws "}"
 attribute = [cardinality ws] [reverseFlag ws] [attributeOperator ws] attributeName ws (expressionComparisonOperator ws expressionConstraintValue / numericComparisonOperator ws numericValue / stringComparisonOperator ws stringValue )
 cardinality = "[" minValue to maxValue "]" 
 minValue = nonNegativeIntegerValue
 to = ".."
 maxValue = nonNegativeIntegerValue / many
 many = "*"
 reverseFlag = "R"
 attributeOperator = descendantOrSelfOf / descendantOf 
 attributeName = conceptReference / wildCard
 expressionConstraintValue = simpleExpressionConstraint / "(" ws (refinedExpressionConstraint / compoundExpressionConstraint) ws ")"
 expressionComparisonOperator = "=" / "!=" 
 numericComparisonOperator = "=" / "!=" / "<=" / "<" / ">=" / ">"
 stringComparisonOperator = "=" / "!=" 
 numericValue = "#" ( decimalValue / integerValue)
 stringValue = QM 1*(anyNonEscapedChar / escapedChar) QM 
 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