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Add Regular Path Patterns to Cypher
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‎cip/1.accepted/CIP2017-02-06-Regular-Path-Patterns.adoc

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= CIP2017-02-06 Regular Path Patterns
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:numbered:
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:toc:
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:toc-placement: macro
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:source-highlighter: codemirror
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*Authors:* Tobias Lindaaker <tobias[email protected]>
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toc::[]
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== Regular Path Patterns
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Above and beyond the types of patterns that can be expressed in Cypher using the normal path syntax, Cypher also supports what amounts to regular expressions over paths.
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Queries of this type are typically referred to as Regular Path Queries (RPQs).
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In Cypher Regular Path Queries are expressed through the use of _Regular Path Patterns_.
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A Regular Path Pattern is defined as:
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• A simple relationship type +
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`()-/:X/-()` denotes a Regular Path Pattern matching relationships of type `X`.
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• A predicate on the labels of a node +
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`()-/(:Z)/-()` denotes a Regular Path Pattern matching nodes with label `Z`.
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• A sequence of Regular Path Patterns +
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`()-/_a_ _b_/-()` denotes a Regular Path Pattern matching first the pattern defined by `_a_`, then the pattern defined by `_b_` (in order left to right).
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• An alternative between Regular Path Patterns +
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`()-/_a_ | _b_/-()` denotes a Regular Path Pattern matching either the pattern defined by `_a_` or the pattern defined by `_b_`.
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• A repetition of a Regular Path Pattern +
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`()-/_a_*/-()` denotes a Regular Path Pattern matching the pattern defined by `_a_` zero or more times. +
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`()-/_a_+/-()` denotes a Regular Path Pattern matching the pattern defined by `_a_` one or more times. +
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`()-/_a_*_x_../-()` denotes a Regular Path Pattern matching the pattern defined by `_a_` `_x_` or more times. +
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`()-/_a_*_x_.._y_/-()` denotes a Regular Path Pattern matching the pattern defined by `_a_` at least `_x_` times and at most `_y_` times.
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• A grouping of a Regular Path Pattern +
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`()-/[_a_]/-()` denotes a grouping of the pattern `_a_`.
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• A specification of direction for a Regular Path Pattern +
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`()-/ _a_ >/-()` denotes that the Regular Path Pattern `_a_` should be interpreted in a left-to-right direction. +
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`()-/< _a_ /-()` denotes that the Regular Path Pattern `_a_` should be interpreted in a right-to-left direction. +
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`()-/< _a_ >/-()` denotes that the Regular Path Pattern `_a_` should be interpreted in any direction.
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• A reference to a Defined Path Predicate +
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`()-/alpha/-()` denotes a reference to a Defined Path Predicate named `alpha`.
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Regular Path Patterns are written similarly to how relationship patterns are written, but enclosed within two slash (`/`) characters instead of brackets (`[]`).
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Contrary to Relationship Patterns, Regular Path Patterns do _not_ allow binding a relationship to a variable.
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In order to bind the matching path to a variable, a Path Assignment should be used, by preceding the path with an identifier and an equals sign (`=`).
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This avoids a problem that existed in the past with repetition of relationships (a syntax that is deprecated with the introduction of Regular Path Patterns), where a relationship variable would bind to a list, making it hard to express predicates over the actual relationships.
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Predicates on parts of a Regular Path Pattern are instead expressed through the use of explicitly defined path predicates.
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=== Syntax
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The syntax of Regular Path Patterns fit into the greater Cypher syntax through `PatternElementChain`.
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----
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PatternElementChain = (RelationshipPattern | RegularPathPattern), NodePattern ;
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RegularPathPattern = (LeftArrowHead, Dash, '/', [RegularPathExpression], '/', Dash, RightArrowHead)
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| (LeftArrowHead, Dash, '/', [RegularPathExpression], '/', Dash)
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| (Dash, '/', [RegularPathExpression], '/', Dash, RightArrowHead)
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| (Dash, '/', [RegularPathExpression], '/', Dash)
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;
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RegularPathExpression = {RegularPathAlternative}- ;
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RegularPathAlternative = RegularPathSequence, {'|', RegularPathSequence} ;
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RegularPathSequence = {RegularPathStar}- ;
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RegularPathStar = RegularPathDirected [('*', [RangeLiteral]) | '+'] ;
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RegularPathDirected = ['<'], RegularPathBase, ['>'] ;
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RegularPathBase = RegularPathRelationship
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| RegularPathAnyRelationship
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| RegularPathNode
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| RegularPathReference
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| '[' RegularPathExpression ']'
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;
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RegularPathRelationship = RelType ;
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RegularPathAnyRelationship = '-' ;
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RegularPathNode = '(' NodeLabels ')' ;
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RegularPathReference = SymbolicName ;
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----
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The `RegularPathReference` is a reference to a Defined Path Predicate.
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These are defined using the following syntax:
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----
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DefinedPathPredicate = 'PATH' PathPredicatePrototype, 'IS', Pattern, [Where] ;
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PathPredicatePrototype = '(', Variable, ')', RegularPathPrototype, '(', Variable, ')' ;
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RegularPathPrototype = (LeftArrowHead, Dash, '/', DefinedPathName, '/', Dash)
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| (Dash, '/', DefinedPathName, '/', Dash, RightArrowHead)
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| (Dash, '/', DefinedPathName, '/', Dash)
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;
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DefinedPathName = SymbolicName ;
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----
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=== Directions
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The direction of relationships matched by a Regular Path Pattern is primarily decided by the directional arrow surrounding the pattern.
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If the arrow points from left to right (i.e. `(left)-/pattern/\->(right)`), the paths described by the pattern are paths in the left-to-right direction, i.e. paths that are _outgoing_ from the node to the left of the pattern, and _incoming_ to the node to the right of the pattern.
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If the arrow points from right to left (i.e. `(left)\<-/pattern/-(right)`), the paths described by the pattern are paths in the right-to-left paths direction, i.e. paths that are _incoming_ to the node to the left of the pattern, and _outgoing_ from the node to the right of the pattern.
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If there are no arrowheads (i.e. `(left)-/pattern/-(right)`), or if both arrowheads are present (i.e. `(left)\<-/pattern/\->(right)`), the paths described by the pattern are paths in either the left-to-right or the right-to-left direction.
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All parts of a Regular Path Pattern will assume the direction of the surrounding arrow, unless the direction is explicitly overridden for that particular part of the pattern.
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A prefix of `<` to part of a pattern overrides the direction of that part to be right-to-left.
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A suffix of `>` to part of a pattern overrides the direction of that part to be left-to-right.
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Both a `<` prefix and a `>` suffix can be used on the same part of the pattern to override the direction of that part to be _either direction_.
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Direction overrides only apply to a single pattern part.
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In order to apply the direction override to multiple parts of the pattern, those parts should be grouped.
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Using both a `<` prefix and a `>` suffix on the same pattern is always the same thing as a disjunction between that pattern with a `<` prefix and that pattern with a `>` suffix.
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This means that `()-/< _a_ >/-()` is the same as `()-/[< _a_] | [_a_ >]/-()`.
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==== Directions and Defined Path Predicates
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When a Defined Path Predicate is referenced the direction of reference is matched with the direction in the declaration of the Defined Path Predicate.
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If the declaration of the Defined Path Predicate is defined left-to-right, but the direction of the reference is right-to-left, the direction of definition of the the Defined Path Predicate is reversed to match that of the reference.
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The same reversal applies if the Defined Path Predicate is defined right-to-left but the direction of the reference is left-to-right.
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If the direction of the reference is _either direction_, the Defined Path Predicate is matched both in its declared direction and its reversed direction.
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If a Defined Path Predicate is declared without a direction, the direction of the reference does not matter, since the direction of the Defined Path Predicate is inherently _any direction_.
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A Defined Path Predicate declared without a direction must have a definition that is equivalent if reversed.
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==== Direction examples
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• `()-/a <[b c] d/\->()` is the same as `()-/a/\->()\<-/b c/-()-/d/\->(d)`, i.e. the direction of the group `b c` has been overridden to be right-to-left in a pattern where the overall direction is left-to-right.
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• `()-/a <b> c/\->()` is the same as `()-/a/\->()-/b/-()-/c/\->()`, i.e. the direction of `b` has been overridden to be _either direction_.
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• `()-/a/-()`, `()-/<a>/-()`, `()-/<a>/\->()`, `()\<-/<a>/-()`, `()\<-/<a>/\->()`, and `()\<-/a/\->()` all mean the same thing: matching `a` in _either direction_.
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Given these Defined Path Predicates:
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[source, cypher]
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----
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PATH (l)-/alpha/->(r) IS (l)-[:X]->()-[:Y]->(r)
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PATH (l)-/beta/->(r) IS (l)<-[:Y]-()<-[:X]-(r)
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PATH (l)-/gamma/-(r) IS (l)-/[:X :Y]> | <[:Y :X]/-(r)
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----
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• `()-/alpha/\->()` is equivalent to `()\<-/beta/-()`
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• `()\<-/alpha/-()` is equivalent to `()-/beta/\->()`
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• `()-/gamma/\->()` is equivalent to `()\<-/gamma/-()`, since both are equivalent to `()-/gamma/-()`
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• `()-/gamma/-()` is equivalent to `()-/alpha/-()`, since `()-/alpha/-()` is the same as `()-/alpha> | <alpha/-()`, which is equivalent to the declaration of `gamma`. +
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It is also equivalent to `()-/<beta | beta>/-()` which is the same as `()-/beta/-()`.
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=== Regular Path Pattern Examples
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The astute reader of the syntax will have noticed that it is possible to express a Regular Path Pattern with an empty path expression:
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[source, cypher]
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----
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MATCH (a)-//-(b)
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----
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This pattern simply states that `a` and `b` must be the same node, and is thus the same as:
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[source, cypher]
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----
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MATCH (a), (b) WHERE a = b
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----
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The same reader will also have noticed that it is possible to define a pattern containing just a relationship type:
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[source, cypher]
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----
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MATCH (a)-/:KNOWS/->(b)
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----
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That pattern is indeed equivalent to the very similar relationship pattern:
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[source, cypher]
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----
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MATCH (a)-[:KNOWS]->(b)
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----
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The main difference being that the variant with a relationship pattern is able to bind that relationship and express further predicates over it.
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The Regular Path Patterns start becoming interesting when larger expressions are put together:
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[source, cypher]
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.Finding someone loved by someone hated by someone you know, transitively
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----
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MATCH (you)-/[:KNOWS :HATES]+ :LOVES/->(someone)
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----
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Note the `+` expressing one or more occurrences of the sequence `KNOWS` followed by `HATES`.
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The direction of each relationship is governed by the overall direction of the Regular Path Pattern.
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It is however possible to explicitly define the direction for a particular part of the pattern.
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This is done by either prefixing that part with `<` for a right-to-left direction or suffix it with `>` for a left-to-right direction.
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It is possible to both prefix the part with `<` and suffix it with `>`, indicating that this part of the pattern matches in any direction.
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[source, cypher]
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.Specifying the direction for different parts of the pattern
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----
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MATCH (you)-/[:KNOWS <:HATES]+ :LOVES/->(someone)
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----
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In the example above we say that the `HATES` relationships should have the opposite direction to the other relationships in the path.
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Through the use of Defined Path Predicates we can express even more predicates over a path:
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[source, cypher]
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.Find a chain of unreciprocated lovers
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----
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MATCH (you)-/unreciprocated_love*/->(someone)
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PATH (a)-/unreciprocated_love/->(b) IS
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(a)-[:LOVES]->(b)
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WHERE NOT EXISTS { (b)-[:LOVES]->(a) }
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----
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Note how there is no colon used for referencing the Defined Path Predicate, the colon is used in Regular Path Patterns only for referencing actual relationship types.
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Sometimes it will be interesting to express a predicate on a node in a Regular Path Pattern.
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This can be achieved by using a Defined Path Predicate where the nodes on both ends are the same:
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[source, cypher]
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.Find friends of friends that are not haters
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----
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MATCH (you)-/:KNOWS not_a_hater :KNOWS/-(friendly_friend_of_friend)
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PATH (x)-/not_a_hater/-(x) IS (x)
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WHERE NOT EXISTS { (x)-[:HATES]->() }
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----
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In the case of a Defined Path Predicate where both nodes are the same, the direction of the predicate is irrelevant.
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In general the direction of a Defined Path Predicate is quite important, and used for mapping the pattern in the predicate into the Regular Path Patterns that reference it.
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The only cases where it is allowed to omit the direction of a Defined Path Predicate is when the defined predicate is reflexive.
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This is obviously the case when both nodes are the same, but it would also be the case when the internal pattern is symmetrical, such as in the following example:
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[source, cypher]
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.Find chains of co-authorship
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----
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MATCH (you)-/co_author*/-(someone)
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PATH (a)-/co_author/-(b) IS
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(a)-[:AUTHORED]->(:Book)<-[:AUTHORED]-(b)
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WHERE a <> b
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----

‎grammar/basic-grammar.xml

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</production>
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<production name="PatternElementChain" rr:inline="true">
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<non-terminal ref="RelationshipPattern"/>
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<alt>
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<non-terminal ref="RelationshipPattern"/>
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<non-terminal ref="RegularPathPattern"/>
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</alt>
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&WS;
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<non-terminal ref="NodePattern"/>
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</production>
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]
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</production>
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<production name="RegularPathPattern">
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<alt>
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<seq>
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<non-terminal ref="LeftArrowHead" rr:title="&lt;"/>
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&WS; <non-terminal ref="Dash" rr:title="-"/>
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&WS; / &WS; <opt><non-terminal ref="RegularPathExpression"/></opt> &WS; /
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&WS; <non-terminal ref="Dash" rr:title="-"/>
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&WS; <non-terminal ref="RightArrowHead" rr:title="&gt;"/>
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</seq>
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<seq>
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<non-terminal ref="LeftArrowHead" rr:title="&lt;"/>
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&WS; <non-terminal ref="Dash" rr:title="-"/>
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&WS; / &WS; <opt><non-terminal ref="RegularPathExpression"/></opt> &WS; /
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&WS; <non-terminal ref="Dash" rr:title="-"/>
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</seq>
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<seq>
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&WS; <non-terminal ref="Dash" rr:title="-"/>
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&WS; / &WS; <opt><non-terminal ref="RegularPathExpression"/></opt> &WS; /
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&WS; <non-terminal ref="Dash" rr:title="-"/>
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&WS; <non-terminal ref="RightArrowHead" rr:title="&gt;"/>
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</seq>
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<seq>
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&WS; <non-terminal ref="Dash" rr:title="-"/>
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&WS; / &WS; <opt><non-terminal ref="RegularPathExpression"/></opt> &WS; /
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&WS; <non-terminal ref="Dash" rr:title="-"/>
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</seq>
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</alt>
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</production>
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<production name="RegularPathExpression">
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<repeat min="1"><non-terminal ref="RegularPathAlternative"/></repeat>
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</production>
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<production name="RegularPathAlternative">
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<non-terminal ref="RegularPathSequence"/>
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<opt>| <non-terminal ref="RegularPathSequence"/></opt>
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</production>
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<production name="RegularPathSequence">
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<repeat min="1"><non-terminal ref="RegularPathStar"/></repeat>
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</production>
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<production name="RegularPathStar">
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<non-terminal ref="RegularPathDirected"/>
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<opt><alt>
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<seq>* <opt><non-terminal ref="RangeLiteral"/></opt></seq>
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+
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</alt></opt>
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</production>
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<production name="RegularPathDirected">
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<opt>&lt;</opt> <non-terminal ref="RegularPathBase"/> <opt>&gt;</opt>
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</production>
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<production name="RegularPathBase">
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<alt>
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<non-terminal ref="RegularPathRelationship"/>
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<non-terminal ref="RegularPathAnyRelationship"/>
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<non-terminal ref="RegularPathNode"/>
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<non-terminal ref="RegularPathReference"/>
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<seq>[ <non-terminal ref="RegularPathExpression"/> ]</seq>
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</alt>
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</production>
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<production name="RegularPathRelationship">
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: <non-terminal ref="RelTypeName"/>
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</production>
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<production name="RegularPathAnyRelationship">
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<non-terminal ref="Dash" rr:title="-"/>
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</production>
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<production name="RegularPathNode">
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( <non-terminal ref="NodeLabels"/> )
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</production>
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<production name="RegularPathReference">
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<non-terminal ref="SymbolicName"/>
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</production>
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<production name="Properties" rr:inline="true">
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<alt>
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<non-terminal ref="MapLiteral"/>

‎grammar/cypher.xml

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</production>
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<production name="RegularQuery" rr:inline="true">
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<!-- TODO: is this where we declare path predicates? -->
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<repeat><non-terminal ref="DefinedPathPredicate"/></repeat>
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<non-terminal ref="SingleQuery"/>
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<repeat>&WS;<non-terminal ref="Union"/></repeat>
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</production>
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<opt>&WS; <non-terminal ref="Where"/></opt>
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</production>
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<production name="DefinedPathPredicate">
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PATH <non-terminal ref="PathPredicatePrototype"/>
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IS <non-terminal ref="Pattern"/>
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<opt><non-terminal ref="Where"/></opt>
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</production>
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<production name="PathPredicatePrototype">
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( <non-terminal ref="Variable"/> )
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<non-terminal ref="RegularPathPrototype"/>
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( <non-terminal ref="Variable"/> )
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</production>
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<production name="RegularPathPrototype">
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<alt>
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<seq>
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<non-terminal ref="LeftArrowHead" rr:title="&lt;"/>
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&WS; <non-terminal ref="Dash" rr:title="-"/>
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&WS; / &WS; <opt><non-terminal ref="DefinedPathName"/></opt> &WS; /
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&WS; <non-terminal ref="Dash" rr:title="-"/>
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</seq>
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<seq>
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&WS; <non-terminal ref="Dash" rr:title="-"/>
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&WS; / &WS; <opt><non-terminal ref="DefinedPathName"/></opt> &WS; /
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&WS; <non-terminal ref="Dash" rr:title="-"/>
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&WS; <non-terminal ref="RightArrowHead" rr:title="&gt;"/>
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</seq>
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<seq>
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&WS; <non-terminal ref="Dash" rr:title="-"/>
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&WS; / &WS; <opt><non-terminal ref="DefinedPathName"/></opt> &WS; /
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&WS; <non-terminal ref="Dash" rr:title="-"/>
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</seq>
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</alt>
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</production>
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<production name="DefinedPathName">
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<non-terminal ref="SymbolicName"/>
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</production>
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<production name="Unwind">
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UNWIND &WS; &expr; &SP; AS &SP; &var;
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</production>

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