An operation of concatenation is introduced for graphs. Then strings
are viewed as expressions denoting graphs, and string languages are
interpreted as graph languages. For a class K of string languages,
Int(K) is the class of all graph languages that are interpretations of
languages from K. For the class REG of regular languages, Int(REG)
might be called the class of regular graph languages; it equals the
class of graph languages generated by linear Hyperedge Replacement
Systems. Two characterizations are given of the largest class K' such
that Int(K') = \Int(K).