To understand basic syntax of LEX specification, built in functions & variable


Problem Statement : To understand basic syntax of LEX specification, built in functions & variable
Theory : Lex helps write programs whose control flow is directed by instances of regular expressions in the input stream. It is well suited for editor-script type transformations and for segmenting input in preparation for a parsing routine.
Lex source is a table of regular expressions and corresponding program fragments. The table is translated to a program which reads an input stream, copying it to an output stream and partitioning the input into strings which match the given expressions. As each such string is recognized the corresponding program fragment is executed. The recognition of the expressions is performed by a deterministic finite automaton generated by Lex. The program fragments written by the user are executed in the order in which the corresponding regular expressions occur in the input stream.
The lexical analysis programs written with Lex accept ambiguous specifications and choose the longest match possible at each input point. If necessary, substantial lookahead is performed on the input, but the input stream will be backed up to the end of the current partition, so that the user has general freedom to manipulate it.
Lex Source.
The general format of Lex source is:
{definitions}
%%
{rules}
%%
{user subroutines}
where the definitions and the user subroutines are often omitted. The second %% is optional, but the first is required to mark the beginning of the rules. The absolute minimum Lex program is thus
%%
(no definitions, no rules) which translates into a program which copies the input to the output unchanged.
In the outline of Lex programs shown above, the rules represent the user's control decisions; they are a table, in which the left column contains regular expressions (see section 3) and the right column contains actions, program fragments to be executed when the expressions are recognized. Thus an individual rule might appear
integer printf("found keyword INT");
to look for the string integer in the input stream and print the message ``found keyword INT'' whenever it appears. In this example the host procedural language is C and the C library function printf is used to print the string. The end of the expression is indicated by the first blank or tab character. If the action is merely a single C expression, it can just be given on the right side of the line; if it is compound, or takes more than a line, it should be enclosed in braces. As a slightly more useful example, suppose it is desired to change a number of words from British to American spelling. Lex rules such as
colour printf("color");
mechanise printf("mechanize");
petrol printf("gas");
would be a start. These rules are not quite enough, since the word petroleum would become gaseum; a way of dealing with this will be described later.
Lex Source Definitions.
Remember the format of the Lex source:
{definitions}
%%
{rules}
%%
{user routines}
So far only the rules have been described. The user needs additional options, though, to define variables for use in his program and for use by Lex. These can go either in the definitions section or in the rules section.
Remember that Lex is turning the rules into a program. Any source not intercepted by Lex is copied into the generated program. There are three classes of such things.
1) Any line which is not part of a Lex rule or action which begins with a blank or tab is copied into the Lex generated program. Such source input prior to the first %% delimiter will be external to any function in the code; if it appears immediately after the first %%, it appears in an appropriate place for declarations in the function written by Lex which contains the actions. This material must look like program fragments, and should precede the first Lex rule. As a side effect of the above, lines which begin with a blank or tab, and which contain a comment, are passed through to the generated program. This can be used to include comments in either the Lex source or the generated code. The comments should follow the host language convention.
2) Anything included between lines containing only %{ and %} is copied out as above. The delimiters are discarded. This format permits entering text like preprocessor statements that must begin in column 1, or copying lines that do not look like programs.
3) Anything after the third %% delimiter, regardless of formats, etc., is copied out after the Lex output.
Definitions intended for Lex are given before the first %% delimiter. Any line in this section not contained between %{ and %}, and begining in column 1, is assumed to define Lex substitution strings. The format of such lines is name translation and it causes the string given as a translation to be associated with the name. The name and translation must be separated by at least one blank or tab, and the name must begin with a letter. The translation can then be called out by the {name} syntax in a rule. Using {D} for the digits and {E} for an exponent field, for example, might abbreviate rules to recognize numbers:
D [0-9]
E [DEde][-+]?{D}+
%%
{D}+ printf("integer");
{D}+"."{D}*({E})? |
{D}*"."{D}+({E})? |
{D}+{E}
Note the first two rules for real numbers; both require a decimal point and contain an optional exponent field, but the first requires at least one digit before the decimal point and the second requires at least one digit after the decimal point. To correctly handle the problem posed by a Fortran expression such as 35.EQ.I, which does not contain a real number, a context-sensitive rule such as
[0-9]+/"."EQ printf("integer");
could be used in addition to the normal rule for integers.
The definitions section may also contain other commands, including the selection of a host language, a character set table, a list of start conditions, or adjustments to the default size of arrays within Lex itself for larger source programs. These possibilities are discussed below under ``Summary of Source Format,'' section 12.
Lex Actions.
When an expression written as above is matched, Lex executes the corresponding action. This section describes some features of Lex which aid in writing actions. Note that there is a default action, which consists of copying the input to the output. This is performed on all strings not otherwise matched. Thus the Lex user who wishes to absorb the entire input, without producing any output, must provide rules to match everything. When Lex is being used with Yacc, this is the normal situation. One may consider that actions are what is done instead of copying the input to the output; thus, in general, a rule which merely copies can be omitted. Also, a character combination which is omitted from the rules and which appears as input is likely to be printed on the output, thus calling attention to the gap in the rules.
One of the simplest things that can be done is to ignore the input. Specifying a C null statement, ; as an action causes this result. A frequent rule is
[ \t\n] ;
which causes the three spacing characters (blank, tab, and newline) to be ignored.
Another easy way to avoid writing actions is the action character |, which indicates that the action for this rule is the action for the next rule. The previous example could also have been written
" "
"\t"
"\n"
with the same result, although in different style. The quotes around \n and \t are not required.
In more complex actions, the user will often want to know the actual text that matched some expression like [a-z]+. Lex leaves this text in an external character array named yytext. Thus, to print the name found, a rule like
[a-z]+ printf("%s", yytext);
will print the string in yytext. The C function printf accepts a format argument and data to be printed; in this case, the format is ``print string'' (% indicating data conversion, and s indicating string type), and the data are the characters in yytext. So this just places the matched string on the output. This action is so common that it may be written as ECHO:
[a-z]+ ECHO;
is the same as the above. Since the default action is just to print the characters found, one might ask why give a rule, like this one, which merely specifies the default action? Such rules are often required to avoid matching some other rule which is not desired. For example, if there is a rule which matches read it will normally match the instances of read contained in bread or readjust; to avoid this, a rule of the form [a-z]+ is needed. This is explained further below.
Sometimes it is more convenient to know the end of what has been found; hence Lex also provides a count yyleng of the number of characters matched. To count both the number of words and the number of characters in words in the input, the user might write [a-zA-Z]+ {words++; chars += yyleng;} which accumulates in chars the number of characters in the words recognized. The last character in the string matched can be accessed by
yytext[yyleng-1]
Occasionally, a Lex action may decide that a rule has not recognized the correct span of characters. Two routines are provided to aid with this situation. First, yymore() can be called to indicate that the next input expression recognized is to be tacked on to the end of this input. Normally, the next input string would overwrite the current entry in yytext. Second, yyless (n) may be called to indicate that not all the characters matched by the currently successful expression are wanted right now. The argument n indicates the number of characters in yytext to be retained. Further characters previously matched are returned to the input. This provides the same sort of lookahead offered by the / operator, but in a different form.
Example: Consider a language which defines a string as a set of characters between quotation (") marks, and provides that to include a " in a string it must be preceded by a \. The regular expression which matches that is somewhat confusing, so that it might be preferable to write
\"[^"]* {
if (yytext[yyleng-1] == '\\')
yymore();
else
... normal user processing
}
which will, when faced with a string such as "abc\"def" first match the five characters "abc\; then the call to yymore() will cause the next part of the string, "def, to be tacked on the end. Note that the final quote terminating the string should be picked up in the code labeled ``normal processing''.
The function yyless() might be used to reprocess text in various circumstances. Consider the C problem of distinguishing the ambiguity of ``=-a''. Suppose it is desired to treat this as ``=- a'' but print a message. A rule might be
=-[a-zA-Z] {
printf("Op (=-) ambiguous\n");
yyless(yyleng-1);
... action for =- ...
}
which prints a message, returns the letter after the operator to the input stream, and treats the operator as ``=-''. Alternatively it might be desired to treat this as ``= -a''. To do this, just return the minus sign as well as the letter to the input:
=-[a-zA-Z] {
printf("Op (=-) ambiguous\n");
yyless(yyleng-2);
... action for = ...
}
will perform the other interpretation. Note that the expressions for the two cases might more easily be written
=-/[A-Za-z]
in the first case and
=/-[A-Za-z]
in the second; no backup would be required in the rule action. It is not necessary to recognize the whole identifier to observe the ambiguity. The possibility of ``=-3'', however, makes
=-/[^ \t\n]
a still better rule.
In addition to these routines, Lex also permits access to the I/O routines it uses. They are:
1) input() which returns the next input character;
2) output(c) which writes the character c on the output; and
3) unput(c) pushes the character c back onto the input stream to be read later by input().
By default these routines are provided as macro definitions, but the user can override them and supply private versions. These routines define the relationship between external files and internal characters, and must all be retained or modified consistently. They may be redefined, to cause input or output to be transmitted to or from strange places, including other programs or internal memory; but the character set used must be consistent in all routines; a value of zero returned by input must mean end of file; and the relationship between unput and input must be retained or the Lex lookahead will not work. Lex does not look ahead at all if it does not have to, but every rule ending in + * ? or $ or containing / implies lookahead. Lookahead is also necessary to match an expression that is a prefix of another expression. See below for a discussion of the character set used by Lex. The standard Lex library imposes a 100 character limit on backup.
Another Lex library routine that the user will sometimes want to redefine is yywrap() which is called whenever Lex reaches an end-of-file. If yywrap returns a 1, Lex continues with the normal wrapup on end of input. Sometimes, however, it is convenient to arrange for more input to arrive from a new source. In this case, the user should provide a yywrap which arranges for new input and returns 0. This instructs Lex to continue processing. The default yywrap always returns 1.
This routine is also a convenient place to print tables, summaries, etc. at the end of a program. Note that it is not possible to write a normal rule which recognizes end-of-file; the only access to this condition is through yywrap. In fact, unless a private version of input() is supplied a file containing nulls cannot be handled, since a value of 0 returned by input is taken to be end-of-file.
Example.
Consider copying an input file while adding 3 to every positive number divisible by 7. Here is a suitable Lex source program
%%
int k;
[0-9]+ {
k = atoi(yytext);
if (k%7 == 0)
printf("%d", k+3);
else
printf("%d",k);
}
to do just that. The rule [0-9]+ recognizes strings of digits; atoi converts the digits to binary and stores the result in k. The operator % (remainder) is used to check whether k is divisible by 7; if it is, it is incremented by 3 as it is written out. It may be objected that this program will alter such input items as 49.63 or X7. Furthermore, it increments the absolute value of all negative numbers divisible by 7. To avoid this, just add a few more rules after the active one, as here:
%%
int k;
-?[0-9]+ {
k = atoi(yytext);
printf("%d",
k%7 == 0 ? k+3 : k);
}
-?[0-9.]+ ECHO;
[A-Za-z][A-Za-z0-9]+ ECHO;
Numerical strings containing a ``.'' or preceded by a letter will be picked up by one of the last two rules, and not changed. The if-else has been replaced by a C conditional expression to save space; the form a?b:c means ``if a then b else c''.

Conclusion : _________________________________________________________

 
Frequently asked questions:
1.What is the use of LEX?
2.What is the general format of LEX?

3.What are classes of LEX?

4.Write a small program using LEX.

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