language, fully developed by Ferruccio Guidi.
For a description of the previous versions of {\MathQL} see: \cite{Gui03}
(version 3), \cite{GS03} (version 2), \cite{Lor02} (version 1).
+
+\subsection{Textual syntax}
+
+The syntax of grammatical productions resembles BNF and POSIX notation:
+
+\begin{itemize}
+
+\item
+\TT{::=} defines a grammatical production by means of a regular expression.
+
+Regular expressions are made of the following elements
+(here \TT{...} is a placeholder):
+
+% \item
+% \TT{.} represents any character between U 0020 and U 007F inclusive;
+
+\item
+\TT{`...`} represents any character in a character set;
+
+\item
+\verb+`^ ...`+ represents any character (U+0020 to U+007E) not in a character
+set;
+
+\item
+\TT{"..."} represents a string to be matched verbatim;
+
+\item
+\GP{...} represents a regular expression defined by a grammatical production;
+
+\item
+\TT{... ...} represents a conjunctive regular expression;
+
+\item
+\TT{... | ...} represents a disjunctive regular expression;
+
+\item
+\TT{[ ... ]?} represents an optional regular expression;
+
+\item
+\TT{[ ... ]+} represents a regular expression to be repeated one or more times;
+
+\item
+\TT{[ ... ]*} represents a regular expression to be repeated zero or more times;
+
+\item
+\TT{[ ... ]} represents a grouped regular expression.
+
+\end{itemize}
+
+\noindent
+{\MathQL} Expressions can contain quoted constant strings with the syntax of
+\figref {StrTS}.%
+\footnote{Note that the first slash of the \GP{path} is not optional as
+in {\MathQL}.3.}
+
+\begin{figure}[ht]
+\begin{footnotesize} \begin{verbatim}
+<dec> ::= '0 - 9'
+<num> ::= <dec> [ <dec> ]*
+<hex> ::= <dec> | 'A - F' | 'a - f'
+<escaped> ::= "u" <hex> <hex> <hex> <hex> | '"' | "\" | "^"
+<string> ::= '"' [ "\" <escaped> "^" | '^ "\^' ]* '"'
+<path> ::= "/" | [ "/" <string> ]+
+\end{verbatim} \end{footnotesize}
+\vskip-1pc
+\caption{Textual syntax of numbers, strings and paths} \label{StrTS}
+\end{figure}
+
+\noindent
+The meaning of the escaped sequences is shown in \figref{EscTS}
+(where $ .... $ is a 4-digit placeholder).
+
+\begin{figure}[ht]
+\begin{footnotesize}
+\begin{center} \begin{tabular}{|l|l|c|}
+\hline {\bf Escape sequence} & {\bf Unicode character} & {\bf Text} \\
+\hline \verb+\u....^+ & U+.... & \\
+\hline \verb+\"^+ & U+0022 & \verb+"+ \\
+\hline \verb+\\^+ & U+005C & \verb+\+ \\
+\hline \verb+\^^+ & U+005E & \verb+^+ \\
+\hline
+\end{tabular} \end{center}
+\end{footnotesize}
+\vskip-1pc
+\caption{Textual syntax of escaped characters} \label{EscTS}
+\end{figure}
+
+{\MathQL} character escaping syntax aims at complying with W3C character model
+for the World Wide Web \cite{W3Ca} which recommends a support for standard
+Unicode characters (U+0000 to U+FFFF) and escape sequences with
+start/end delimiters.
+In particular {\MathQL} escape delimiters (backslash and caret) are chosen
+among the {\em unwise} characters for URI references (see \cite{URI}) because
+URI references are the natural content of constant strings and these
+characters should not be so frequent in them.
+
+Query expressions can contain variables for {\av}'s (production \GP{avar})
+and variables for {\av} sets, {\ie} for query results (production \GP{svar})
+according to the syntax of \figref{VarTS}.
+
+\begin{figure}[ht]
+\begin{footnotesize} \begin{verbatim}
+<alpha> ::= [ 'A - Z' | 'a - z' | `_` ]+
+<id> ::= <alpha> [ <alpha> | <dec> ]*
+<avar> ::= "@" <id>
+<svar> ::= "$" <id>
+\end{verbatim}\end{footnotesize} %$
+\vskip-1pc
+\caption{Textual syntax of variables} \label{VarTS}
+\end{figure}
+
+\noindent
+The syntax of query expressions (production \GP{query}) is described in
+\figref{QueryTS}.
+
+\begin{figure}[ht]
+\begin{footnotesize} \begin{verbatim}
+<qualifier> ::= [ "inverse" ]? [ "sub" | "super" ]? <path>
+<main> ::= [ "main" <path> ]?
+<cons> ::= <path> [ "in" | "match" ] <query>
+<istrue> ::= [ "istrue" <cons> [ "," <cons> ]* ]?
+<isfalses> ::= [ "isfalse" <cons> [ "," <cons> ]* ]*
+<exp> ::= <path> [ "as" <path> ]?
+<sec> ::= [ "attr" <exp> [ "," <exp> ]* ]?
+<opt_args> ::= <main> <istrue> <isfalses> <sec>
+<source> ::= [ "pattern" ]? <query>
+<paths> ::= [ <path> [ "," <path> ]* ]?
+<query> ::= "(" <query> ")" | <string> | "[" <xavs> "]"
+ | "property" <qualifier> <opt_args> "of" <source>
+ | "let" <svar> "=" <query> "in" <query>
+ | <query> ";;" <query> | <svar> | <avar>
+ | "ex" <query> | <avar> "." <path>
+ | "add" [ "distr" ]? [ <xgroups> | <avar> ] "in" <query>
+ | "for" <avar> "in" <query> [ "sup" | "inf" ] <query>
+ | "while" <query> [ "sup" | "inf" ] <query>
+ | <path> "{" <paths> "}" "{" <queries> "}"
+ | "gen" <path> [ "{" <queries> "}" | "in" <query> ]
+<queries> ::= [ <query> [ "," <query> ]* ]?
+<xattr> ::= <path> "=" <query>
+<xgroup> ::= "{" <xattr> [ ";" <xattr> ]* "}"
+<xgroups> ::= <xgroup> [ "," <xgroup> ]*
+<xav> ::= <string> [ "attr" <xgroups> ]?
+<xavs> ::= [ <xav> [ ";" <xav> ]* ]?
+\end{verbatim} \end{footnotesize}
+\vskip-1pc
+\caption{Textual syntax of queries} \label{QueryTS}
+\end{figure}
+
+\noindent
+The syntax of result expressions (production \GP{avs}) is described in
+\figref{ResultTS}.
+
+\begin{figure}[ht]
+\begin{footnotesize} \begin{verbatim}
+<attr> ::= <path> "=" "{" [ <string> [ "," <string> ]* ]? "}"
+<group> ::= "{" <attr> [ ";" <attr> ]* "}"
+<av> ::= <string> [ "attr" <group> [ "," <group> ]* ]?
+<avs> ::= [ <av> [ ";" <av> ]* ]?
+\end{verbatim} \end{footnotesize}
+\vskip-1pc
+\caption{Textual syntax of results} \label{ResultTS}
+\end{figure}
+
+\xcomment {
+
+\begin{figure}[ht]
+\begin{footnotesize} \begin{verbatim}
+ | "select" <avar> "from" <query> "where" <query>
+\end{verbatim} \end{footnotesize}
+\vskip-1pc
+\caption{Textual syntax of basic query extensions} \label{BasicTS}
+\end{figure}
+
+}
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