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+\chapter{Lexical analysis}
+
+A Python program is read by a {\em parser}.  Input to the parser is a
+stream of {\em tokens}, generated by the {\em lexical analyzer}.  This
+chapter describes how the lexical analyzer breaks a file into tokens.
+\index{lexical analysis}
+\index{parser}
+\index{token}
+
+\section{Line structure}
+
+A Python program is divided in a number of logical lines.  The end of
+a logical line is represented by the token NEWLINE.  Statements cannot
+cross logical line boundaries except where NEWLINE is allowed by the
+syntax (e.g. between statements in compound statements).
+\index{line structure}
+\index{logical line}
+\index{NEWLINE token}
+
+\subsection{Comments}
+
+A comment starts with a hash character (\verb\#\) that is not part of
+a string literal, and ends at the end of the physical line.  A comment
+always signifies the end of the logical line.  Comments are ignored by
+the syntax.
+\index{comment}
+\index{logical line}
+\index{physical line}
+\index{hash character}
+
+\subsection{Line joining}
+
+Two or more physical lines may be joined into logical lines using
+backslash characters (\verb/\/), as follows: when a physical line ends
+in a backslash that is not part of a string literal or comment, it is
+joined with the following forming a single logical line, deleting the
+backslash and the following end-of-line character.  For example:
+\index{physical line}
+\index{line joining}
+\index{backslash character}
+%
+\begin{verbatim}
+month_names = ['Januari', 'Februari', 'Maart',     \
+               'April',   'Mei',      'Juni',      \
+               'Juli',    'Augustus', 'September', \
+               'Oktober', 'November', 'December']
+\end{verbatim}
+
+\subsection{Blank lines}
+
+A logical line that contains only spaces, tabs, and possibly a
+comment, is ignored (i.e., no NEWLINE token is generated), except that
+during interactive input of statements, an entirely blank logical line
+terminates a multi-line statement.
+\index{blank line}
+
+\subsection{Indentation}
+
+Leading whitespace (spaces and tabs) at the beginning of a logical
+line is used to compute the indentation level of the line, which in
+turn is used to determine the grouping of statements.
+\index{indentation}
+\index{whitespace}
+\index{leading whitespace}
+\index{space}
+\index{tab}
+\index{grouping}
+\index{statement grouping}
+
+First, tabs are replaced (from left to right) by one to eight spaces
+such that the total number of characters up to there is a multiple of
+eight (this is intended to be the same rule as used by {\UNIX}).  The
+total number of spaces preceding the first non-blank character then
+determines the line's indentation.  Indentation cannot be split over
+multiple physical lines using backslashes.
+
+The indentation levels of consecutive lines are used to generate
+INDENT and DEDENT tokens, using a stack, as follows.
+\index{INDENT token}
+\index{DEDENT token}
+
+Before the first line of the file is read, a single zero is pushed on
+the stack; this will never be popped off again.  The numbers pushed on
+the stack will always be strictly increasing from bottom to top.  At
+the beginning of each logical line, the line's indentation level is
+compared to the top of the stack.  If it is equal, nothing happens.
+If it is larger, it is pushed on the stack, and one INDENT token is
+generated.  If it is smaller, it {\em must} be one of the numbers
+occurring on the stack; all numbers on the stack that are larger are
+popped off, and for each number popped off a DEDENT token is
+generated.  At the end of the file, a DEDENT token is generated for
+each number remaining on the stack that is larger than zero.
+
+Here is an example of a correctly (though confusingly) indented piece
+of Python code:
+
+\begin{verbatim}
+def perm(l):
+        # Compute the list of all permutations of l
+
+    if len(l) <= 1:
+                  return [l]
+    r = []
+    for i in range(len(l)):
+             s = l[:i] + l[i+1:]
+             p = perm(s)
+             for x in p:
+              r.append(l[i:i+1] + x)
+    return r
+\end{verbatim}
+
+The following example shows various indentation errors:
+
+\begin{verbatim}
+    def perm(l):                        # error: first line indented
+    for i in range(len(l)):             # error: not indented
+        s = l[:i] + l[i+1:]
+            p = perm(l[:i] + l[i+1:])   # error: unexpected indent
+            for x in p:
+                    r.append(l[i:i+1] + x)
+                return r                # error: inconsistent dedent
+\end{verbatim}
+
+(Actually, the first three errors are detected by the parser; only the
+last error is found by the lexical analyzer --- the indentation of
+\verb\return r\ does not match a level popped off the stack.)
+
+\section{Other tokens}
+
+Besides NEWLINE, INDENT and DEDENT, the following categories of tokens
+exist: identifiers, keywords, literals, operators, and delimiters.
+Spaces and tabs are not tokens, but serve to delimit tokens.  Where
+ambiguity exists, a token comprises the longest possible string that
+forms a legal token, when read from left to right.
+
+\section{Identifiers}
+
+Identifiers (also referred to as names) are described by the following
+lexical definitions:
+\index{identifier}
+\index{name}
+
+\begin{verbatim}
+identifier:     (letter|"_") (letter|digit|"_")*
+letter:         lowercase | uppercase
+lowercase:      "a"..."z"
+uppercase:      "A"..."Z"
+digit:          "0"..."9"
+\end{verbatim}
+
+Identifiers are unlimited in length.  Case is significant.
+
+\subsection{Keywords}
+
+The following identifiers are used as reserved words, or {\em
+keywords} of the language, and cannot be used as ordinary
+identifiers.  They must be spelled exactly as written here:
+\index{keyword}
+\index{reserved word}
+
+\begin{verbatim}
+and        del        for        in         print
+break      elif       from       is         raise
+class      else       global     not        return
+continue   except     if         or         try
+def        finally    import     pass       while
+\end{verbatim}
+
+%	# This Python program sorts and formats the above table
+%	import string
+%	l = []
+%	try:
+%		while 1:
+%			l = l + string.split(raw_input())
+%	except EOFError:
+%		pass
+%	l.sort()
+%	for i in range((len(l)+4)/5):
+%		for j in range(i, len(l), 5):
+%			print string.ljust(l[j], 10),
+%		print
+
+\section{Literals} \label{literals}
+
+Literals are notations for constant values of some built-in types.
+\index{literal}
+\index{constant}
+
+\subsection{String literals}
+
+String literals are described by the following lexical definitions:
+\index{string literal}
+
+\begin{verbatim}
+stringliteral:  "'" stringitem* "'"
+stringitem:     stringchar | escapeseq
+stringchar:     <any ASCII character except newline or "\" or "'">
+escapeseq:      "'" <any ASCII character except newline>
+\end{verbatim}
+\index{ASCII}
+
+String literals cannot span physical line boundaries.  Escape
+sequences in strings are actually interpreted according to rules
+similar to those used by Standard C.  The recognized escape sequences
+are:
+\index{physical line}
+\index{escape sequence}
+\index{Standard C}
+\index{C}
+
+\begin{center}
+\begin{tabular}{|l|l|}
+\hline
+\verb/\\/	& Backslash (\verb/\/) \\
+\verb/\'/	& Single quote (\verb/'/) \\
+\verb/\a/	& ASCII Bell (BEL) \\
+\verb/\b/	& ASCII Backspace (BS) \\
+%\verb/\E/	& ASCII Escape (ESC) \\
+\verb/\f/	& ASCII Formfeed (FF) \\
+\verb/\n/	& ASCII Linefeed (LF) \\
+\verb/\r/	& ASCII Carriage Return (CR) \\
+\verb/\t/	& ASCII Horizontal Tab (TAB) \\
+\verb/\v/	& ASCII Vertical Tab (VT) \\
+\verb/\/{\em ooo}	& ASCII character with octal value {\em ooo} \\
+\verb/\x/{\em xx...}	& ASCII character with hex value {\em xx...} \\
+\hline
+\end{tabular}
+\end{center}
+\index{ASCII}
+
+In strict compatibility with Standard C, up to three octal digits are
+accepted, but an unlimited number of hex digits is taken to be part of
+the hex escape (and then the lower 8 bits of the resulting hex number
+are used in all current implementations...).
+
+All unrecognized escape sequences are left in the string unchanged,
+i.e., {\em the backslash is left in the string.}  (This behavior is
+useful when debugging: if an escape sequence is mistyped, the
+resulting output is more easily recognized as broken.  It also helps a
+great deal for string literals used as regular expressions or
+otherwise passed to other modules that do their own escape handling.)
+\index{unrecognized escape sequence}
+
+\subsection{Numeric literals}
+
+There are three types of numeric literals: plain integers, long
+integers, and floating point numbers.
+\index{number}
+\index{numeric literal}
+\index{integer literal}
+\index{plain integer literal}
+\index{long integer literal}
+\index{floating point literal}
+\index{hexadecimal literal}
+\index{octal literal}
+\index{decimal literal}
+
+Integer and long integer literals are described by the following
+lexical definitions:
+
+\begin{verbatim}
+longinteger:    integer ("l"|"L")
+integer:        decimalinteger | octinteger | hexinteger
+decimalinteger: nonzerodigit digit* | "0"
+octinteger:     "0" octdigit+
+hexinteger:     "0" ("x"|"X") hexdigit+
+
+nonzerodigit:   "1"..."9"
+octdigit:       "0"..."7"
+hexdigit:        digit|"a"..."f"|"A"..."F"
+\end{verbatim}
+
+Although both lower case `l' and upper case `L' are allowed as suffix
+for long integers, it is strongly recommended to always use `L', since
+the letter `l' looks too much like the digit `1'.
+
+Plain integer decimal literals must be at most $2^{31} - 1$ (i.e., the
+largest positive integer, assuming 32-bit arithmetic).  Plain octal and
+hexadecimal literals may be as large as $2^{32} - 1$, but values
+larger than $2^{31} - 1$ are converted to a negative value by
+subtracting $2^{32}$.  There is no limit for long integer literals.
+
+Some examples of plain and long integer literals:
+
+\begin{verbatim}
+7     2147483647                        0177    0x80000000
+3L    79228162514264337593543950336L    0377L   0x100000000L
+\end{verbatim}
+
+Floating point literals are described by the following lexical
+definitions:
+
+\begin{verbatim}
+floatnumber:    pointfloat | exponentfloat
+pointfloat:     [intpart] fraction | intpart "."
+exponentfloat:  (intpart | pointfloat) exponent
+intpart:        digit+
+fraction:       "." digit+
+exponent:       ("e"|"E") ["+"|"-"] digit+
+\end{verbatim}
+
+The allowed range of floating point literals is
+implementation-dependent.
+
+Some examples of floating point literals:
+
+\begin{verbatim}
+3.14    10.    .001    1e100    3.14e-10
+\end{verbatim}
+
+Note that numeric literals do not include a sign; a phrase like
+\verb\-1\ is actually an expression composed of the operator
+\verb\-\ and the literal \verb\1\.
+
+\section{Operators}
+
+The following tokens are operators:
+\index{operators}
+
+\begin{verbatim}
++       -       *       /       %
+<<      >>      &       |       ^       ~
+<       ==      >       <=      <>      !=      >=
+\end{verbatim}
+
+The comparison operators \verb\<>\ and \verb\!=\ are alternate
+spellings of the same operator.
+
+\section{Delimiters}
+
+The following tokens serve as delimiters or otherwise have a special
+meaning:
+\index{delimiters}
+
+\begin{verbatim}
+(       )       [       ]       {       }
+;       ,       :       .       `       =
+\end{verbatim}
+
+The following printing ASCII characters are not used in Python.  Their
+occurrence outside string literals and comments is an unconditional
+error:
+\index{ASCII}
+
+\begin{verbatim}
+@       $       "       ?
+\end{verbatim}
+
+They may be used by future versions of the language though!