REGEXP(5) SysV REGEXP(5)
NAME
regexp - regular expression compile and match routines
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 (instring, expbuf, endbuf, eof)
char *instring, *expbuf, *endbuf;
int eof;
int step (string, expbuf)
char *string, *expbuf;
extern char *loc1, *loc2, *locs;
extern int circf, sed, nbra;
DESCRIPTION
This page describes general-purpose regular expression matching routines
in the form of ed(1), defined in <regexp.h> . Programs such as ed(1),
sed(1), grep(1), bs(1), expr(1), etc., which perform regular expression
matching use this source file. In this way, only this file need be
changed to maintain regular expression compatibility.
The interface to this file is unpleasantly complex. Programs that
include this file must have the following five macros declared before the
"#include <regexp.h>" statement. These macros are used by the compile
routine.
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
routine. 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 routine.
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 \{ \}.
49 [ ] imbalance.
50 Regular expression overflow.
The syntax of the compile routine is as follows:
compile(instring, expbuf, endbuf, eof)
The first parameter instring is never used explicitly by the compile
routine but is useful for programs that pass down different pointers to
input characters. It is sometimes used in the INIT declaration (see
below). Programs which 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 which 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(1).
There are other functions in this file which 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
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.
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(1) and sed(1) 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(1):
#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>
...
(void) compile(*argv, expbuf, &expbuf[ESIZE], '\0');
...
if (step(linebuf, expbuf))
succeed();
SEE ALSO
ed(1), expr(1), grep(1), sed(1) in the SysV Command Reference.