regexp(0) CLIX regexp(0)
NAME
regexp - Regular expression compile and match functions
SYNOPSIS
#define INIT declarations
#define GETC() getc code
#define PEEKC() peekc code
#define UNGETC(c) ungetc code
#define RETURN(pointer) return code
#define ERROR(val) error code
#include <regexp.h>
char *compile(
char *instring ,
char *expbuf ,
char *endbuf ,
int eof );
int step(
char *string ,
char *expbuf );
extern char *loc1;
extern char *loc2;
extern char *locs;
extern int circf;
extern int sed;
extern int nbra;
DESCRIPTION
This page describes general-purpose regular expression matching functions
in the form of ed(1), defined in <regexp.h>. Commands such as ed, sed,
grep, bs, expr, and so on, which perform regular expression matching use
this source file. In this way, only this file need be changed to maintain
regular expression compatibility.
Programs that include this file must have the following five macros
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regexp(0) CLIX regexp(0)
declared before the #include <regexp.h> statement. These macros are used
by the compile() function.
GETC() Return the value of the next character in the regular expression
pattern. Successive calls to GETC() should return successive
characters of the regular expression.
PEEKC()
Return the next character in the regular expression. Successive
calls to PEEKC() should return the same character, which should
also be the next character returned by GETC().
UNGETC(c)
Cause the argument c to be returned by the next call to GETC() (and
PEEKC()). No more that one character of pushback is ever needed
and this character is guaranteed to be the last character read by
GETC(). The value of the macro UNGETC(c) is always ignored.
RETURN(pointer)
This macro is used on normal exit of the compile() function. The
value of the argument pointer is a pointer to the character after
the last character of the compiled regular expression. This is
useful to programs which have memory allocation to manage.
ERROR(val)
This is the abnormal return from the compile() function. The
argument val is an error number (see table below for meanings).
This call should never return.
ERROR MEANING
11 Range endpoint too large.
16 Bad number.
25 ``\digit'' out of range.
36 Illegal or missing delimiter.
41 No remembered search string.
42 \( \) imbalance.
43 Too many \(.
44 More than 2 numbers given in \{ \}.
45 } expected after \.
46 First number exceeds second in \{ \}.
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49 [ ] imbalance.
50 Regular expression overflow.
The syntax of the compile() function is as follows:
compile(instring, expbuf, endbuf, eof)
The first parameter instring is never used explicitly by the compile()
function but is useful programs that pass down different pointers to input
characters. It is sometimes used in the INIT declaration (see below).
Programs that call functions to input characters or have characters in an
external array can pass down a value of ((char *) 0) for this parameter.
The next parameter expbuf is a character pointer. It points to the place
where the compiled regular expression will be placed.
The parameter endbuf is one more than the highest address where the
compiled regular expression may be placed. If the compiled expression
cannot fit in (endbuf - expbuf) bytes, a call to ERROR(50) is made.
The parameter eof is the character that marks the end of the regular
expression. For example, in ed(1), this character is usually a /.
Each program that includes this file must have a #define statement for
INIT. This definition will be placed right after the declaration for the
function compile() and the opening curly brace ({). It is used for
dependent declarations and initializations. Most often it is used to set
a register variable to point the beginning of the regular expression so
that this register variable can be used in the declarations for GETC(),
PEEKC() and UNGETC(). Otherwise it can be used to declare external
variables that might be used by GETC(), PEEKC() and UNGETC(). See the
example below of the declarations taken from grep.
There are other functions in this file that perform actual regular
expression matching, one of which is the function step(). The call to
step() is as follows:
step(string, expbuf)
The first parameter to step() is a pointer to a string of characters to be
checked for a match. This string should be null terminated.
The second parameter expbuf is the compiled regular expression which was
obtained by a call of the function compile().
The function step() returns nonzero if the given string matches the
regular expression, and zero if the expressions do not match. If there is
a match, two external character pointers are set as a side effect to the
call to step(). The variable set in step() is loc1. This is a pointer to
the first character that matched the regular expression. The variable
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regexp(0) CLIX regexp(0)
loc2, which is set by the function advance(), points to the character
after the last character that matches the regular expression. Thus if the
regular expression matches the entire line, loc1 will point to the first
character of string and loc2 will point to the null at the end of string.
The step() uses the external variable circf which is set by compile() if
the regular expression begins with ^. If this is set then step() will try
to match the regular expression to the beginning of the string only. If
more than one regular expression is to be compiled before the first is
executed the value of circf should be saved for each compiled expression
and circf should be set to that saved value before each call to step().
The function advance() is called from step() with the same arguments as
step(). The purpose of step() is to step through the string argument and
call advance() until advance() returns nonzero indicating a match or until
the end of string is reached. If one wants to constrain string to the
beginning of the line in all cases, step() need not be called; simply call
advance().
When advance() encounters a * or \{ \} sequence in the regular expression,
it will advance its pointer to the string to be matched as far as possible
and will recursively call itself trying to match the rest of the string to
the rest of the regular expression. As long as there is no match,
advance() will back up along the string until it finds a match or reaches
the point in the string that initially matched the * or \{ \}. It is
sometimes desirable to stop this backing up before the initial point in
the string is reached. If the external character pointer locs is equal to
the point in the string at sometime during the backing up process,
advance() will break out of the loop that backs up and will return zero.
This is used by ed and sed for substitutions done globally (not just the
first occurrence, but the whole line) so, for example, expressions like
s/y*//g do not loop forever.
The additional external variables sed and nbra are used for special
purposes.
EXAMPLES
The following is an example of how the regular expression macros and calls
look from grep:
#define INIT register char *sp = instring;
#define GETC() (*sp++)
#define PEEKC() (*sp)
#define UNGETC(c) (--sp)
#define RETURN(c) return;
#define ERROR(c) regerr();
#include <regexp.h>
...
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regexp(0) CLIX regexp(0)
(void) compile(*argv, expbuf, &expbuf[ESIZE], '\0';
...
if (step(linebuf, expbuf))
succeed();
RELATED INFORMATION
Commands: ed(1), expr(1), grep(1), sed(1)
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