The keywords node, edge, graph, digraph, subgraph, and strict are case-independent.Note also that the allowed compass point values are not keywords, sothese strings can be used elsewhere as ordinary identifiers and, conversely,the parser will actually accept any identifier.

An ID is one of the following:

• Any string of alphabetic ([a-zA-Z200-377]) characters, underscores ('_') ordigits([0-9]), not beginning with a digit;
• a numeral [-]^?(.[0-9]⁺ | [0-9]⁺(.[0-9]^*)^? );
• any double-quoted string ('...') possibly containing escaped quotes (')¹;
• an HTML string (<...>).

An ID is just a string; the lack of quote characters in the first twoforms is just for simplicity. There is no semantic difference betweenabc_2 and 'abc_2', or between 2.34 and '2.34'. Obviously, to use a keyword as an ID, it must be quoted.Note that, in HTML strings, anglebrackets must occur in matched pairs, and newlines and other formatting whitespacecharacters are allowed.In addition, the content must be legal XML, so that the special XMLescape sequences for ', &, <, and > may be necessaryin order to embed these characters in attribute values or raw text.As an ID, an HTML string can be any legal XML string. However, if usedas a label attribute, it is interpreted specially and must follow the syntaxfor HTML-like labels.

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Both quoted strings and HTML strings are scanned as a unit, soany embedded comments will be treated as part of the strings.

An edgeop is -> in directed graphs and -- inundirected graphs.

The language supports C++-style comments: /* */ and //.In addition, a line beginning with a ‘#’ character is considered a lineoutput from a C preprocessor (e.g., # 34 to indicate line 34 ) and discarded.

Semicolons and commas aid readability but are not required.Also, any amount of whitespace may be inserted between terminals.

As another aid for readability, dot allows double-quoted strings tospan multiple physical lines using the standard C convention of abackslash immediately preceding a newline character². In addition, double-quoted strings can be concatenated using a ‘+’ operator.As HTML strings can contain newline characters, which are used solely for formatting, the language does not allow escaped newlines orconcatenation operators to be used within them.

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Subgraphs and Clusters

Subgraphs play three roles in Graphviz. First, a subgraph can be used to represent graph structure, indicating that certain nodes and edges should be grouped together. This is the usual role for subgraphs and typically specifies semantic information about the graph components. It can also provide a convenient shorthand for edges. An edge statement allowsa subgraph on both the left and right sides of the edge operator.When this occurs, an edge is created from every node on the left to every nodeon the right. For example, the specification

is equivalent to

In the second role, a subgraph can provide a context for setting attributes.For example, a subgraph could specify that blue is the default color for all nodes defined in it. In the context of graph drawing, a more interesting example is:

This (anonymous) subgraph specifies that the nodes A, B and C should all be placed on the same rank if drawn using dot.

The third role for subgraphs directly involves how the graphwill be laid out by certain layout engines. If the name of the subgraph begins with cluster, Graphviz notes the subgraph asa special cluster subgraph. If supported, the layout engine willdo the layout so that the nodes belonging to the cluster are drawn together, with the entire drawing of the cluster contained within a bounding rectangle. Note that, for good and bad, cluster subgraphs are not part of theDOT language, but solely a syntactic convention adhered to bycertain of the layout engines.

Lexical and Semantic Notes

A graph must be specified as either a digraph or a graph.Semantically, this indicates whether or not there is a natural direction fromone of the edge’s nodes to the other. Lexically, a digraph must specify an edge using the edge operator ->while a undirected graph must use --.Operationally, the distinction is used to define different default renderingattributes. For example, edges in a digraph will be drawn, by default, withan arrowhead pointing to the head node. For ordinary graphs, edges are drawnwithout any arrowheads by default.

A graph may also be described as strict.This forbids the creation of multi-edges, i.e., there can be at most one edge with a given tail node and head node in the directed case. For undirected graphs, there can be at most oneedge connected to the same two nodes. Subsequent edge statements usingthe same two nodes will identify the edge with the previously defined oneand apply any attributes given in the edge statement.For example, the graph

will have a single edge connecting nodes a and b,whose color is blue.

If a default attribute isdefined using a node, edge, or graph statement,or by an attribute assignment not attached to a node or edge, any object of theappropriate type defined afterwards will inherit this attribute value.This holds until the default attribute is set to a new value, from whichpoint the new value is used. Objects defined before a default attributeis set will have an empty string value attached to the attribute oncethe default attribute definition is made.

Note, in particular, that a subgraph receives the attribute settings ofits parent graph at the time of its definition. This can be useful; forexample, one can assign a font to the root graph and all subgraphs willalso use the font. For some attributes, however, this property isundesirable. If one attaches a label to the root graph, it is probablynot the desired effect to have the label used by all subgraphs. Ratherthan listing the graph attribute at the top of the graph, and theresetting the attribute as needed in the subgraphs, one can simply deferthe attribute definition in the graph until the appropriate subgraphshave been defined.

If an edge belongs to a cluster, its endpoints belong to that cluster.Thus, where you put an edge can effect a layout, as clusters are sometimeslaid out recursively.

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There are certain restrictions on subgraphs and clusters. First, atpresent, the names of a graph and it subgraphs share the same namespace.Thus, each subgraph must have a unique name. Second, although nodescan belong to any number of subgraphs, it is assumed clusters forma strict hierarchy when viewed as subsets of nodes and edges.

Character encodings

The DOT language assumes at least the ASCII character set.Quoted strings, both ordinary and HTML-like, may contain non-ASCII characters.In most cases, these strings are uninterpreted: they simply serve asunique identifiers or values passed through untouched. Labels, however,are meant to be displayed, which requires that the software be able tocompute the size of the text and determine the appropriate glyphs. For this, it needs to know what character encoding is used.

By default, DOT assumes the UTF-8 character encoding. It also acceptsthe Latin1 (ISO-8859-1) character set, assuming the input graph usesthe charset attribute tospecify this. For graphs using othercharacter sets, there are usually programs, such as iconv, whichwill translate from one character set to another.

Another way to avoid non-ASCII characters in labels is to use HTML entitiesfor special characters. During label evaluation, these entities aretranslated into the underlying character. Thistable shows the supported entities, with their Unicode value, a typicalglyph, and the HTML entity name. Thus, to include a lower-case Greek betainto a string, one can use the ASCII sequence β.In general, one should only use entities that are allowed in the outputcharacter set, and for which there is a glyph in the font.

1. In quoted strings in DOT, the only escaped character is double-quote'. That is, in quoted strings, the dyad ' is converted to '; all othercharacters are left unchanged. In particular, remains . Layoutengines may apply additional escape sequences.
2. Previous to 2.30, the language allowed escaped newlines to be used anywhere outsideof HTML strings. The new lex-based scanner makes this difficult to implement. Given theperceived lack of usefulness of this generality, we have restricted this feature todouble-quoted strings, where it can actually be helpful.