regexp(5) UNIX System V regexp(5)
NAME
regexp: compile, step, advance - regular expression compile and match
routines
SYNOPSIS
#define INIT declarations
#define GETC(void) getc code
#define PEEKC(void) peekc code
#define UNGETC(void) ungetc code
#define RETURN(ptr) 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);
int advance(char *string, char *expbuf);
extern char *loc1, *loc2, *locs;
DESCRIPTION
These functions are general purpose regular expression matching routines
to be used in programs that perform regular expression matching. These
functions are defined by the regexp.h header file.
The functions step and advance do pattern matching given a character
string and a compiled regular expression as input.
The function compile takes as input a regular expression as defined below
and produces a compiled expression that can be used with step or advance.
A regular expression specifies a set of character strings. A member of
this set of strings is said to be matched by the regular expression.
Some characters have special meaning when used in a regular expression;
other characters stand for themselves.
The regular expressions available for use with the regexp functions are
constructed as follows:
Expression Meaning
c the character c where c is not a special character.
\c the character c where c is any character, except a digit in
the range 1-9.
^ the beginning of the line being compared.
$ the end of the line being compared.
. any character in the input.
[s] any character in the set s, where s is a sequence of
characters and/or a range of characters, for example, [c-c].
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[^s] any character not in the set s, where s is defined as above.
r* zero or more successive occurrences of the regular expression
r. The longest leftmost match is chosen.
rx the occurrence of regular expression r followed by the
occurrence of regular expression x. (Concatenation)
r\{m,n\} any number of m through n successive occurrences of the
regular expression r. The regular expression r\{m\} matches
exactly m occurrences; r\{m,\} matches at least m
occurrences.
\(r\) the regular expression r. When \n (where n is a number
greater than zero) appears in a constructed regular
expression, it stands for the regular expression x where x is
the nth regular expression enclosed in \( and \) that
appeared earlier in the constructed regular expression. For
example, \(r\)x\(y\)z\2 is the concatenation of regular
expressions rxyzy.
Characters that have special meaning except when they appear within
square brackets ([]) or are preceded by \ are: ., *, [, \. Other
special characters, such as $ have special meaning in more restricted
contexts.
The character ^ at the beginning of an expression permits a successful
match only immediately after a newline, and the character $ at the end of
an expression requires a trailing newline.
Two characters have special meaning only when used within square
brackets. The character - denotes a range, [c-c], unless it is just
after the open bracket or before the closing bracket, [-c] or [c-] in
which case it has no special meaning. When used within brackets, the
character ^ has the meaning complement of if it immediately follows the
open bracket (example: [^c]); elsewhere between brackets (example: [c^])
it stands for the ordinary character ^.
The special meaning of the \ operator can be escaped only by preceding it
with another \, for example, \\.
Programs must have the following five macros declared before the #include
regexp.h statement. These macros are used by the compile routine. The
macros GETC, PEEKC, and UNGETC operate on the regular expression given as
input to compile.
GETC This macro returns the value of the next character (byte)
in the regular expression pattern. Successive calls to
GETC should return successive characters of the regular
expression.
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regexp(5) UNIX System V regexp(5)
PEEKC This macro returns the next character (byte) in the
regular expression. Immediately successive calls to PEEKC
should return the same character, which should also be the
next character returned by GETC.
UNGETC This macro causes the argument c to be returned by the
next call to GETC and PEEKC. No more than one character
of pushback is ever needed and this character is
guaranteed to be the last character read by GETC. The
return value of the macro UNGETC(c) is always ignored.
RETURN(ptr) This macro is used on normal exit of the compile routine.
The value of the argument ptr 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 macro is the abnormal return from the compile
routine. The argument val is an error number [see ERRORS
below for meanings]. This call should never return.
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. This character is usually a /.
Each program that includes the regexp.h header file must have a #define
statement for INIT. It is used for dependent declarations and
initializations. Most often it is used to set a register variable to
point to 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 EXAMPLE below.]
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regexp(5) UNIX System V regexp(5)
The first parameter to the step and advance functions 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 to the function compile.
The function step returns non-zero if some substring of string matches
the regular expression in expbuf and zero if there is no match. If there
is a match, two external character pointers are set as a side effect to
the call to step. The variable loc1 points to the first character that
matched the regular expression; the variable loc2 points to the character
after the last character that matches the regular expression. Thus if
the regular expression matches the entire input string, loc1 will point
to the first character of string and loc2 will point to the null at the
end of string.
The function advance returns non-zero if the initial substring of string
matches the regular expression in expbuf. If there is a match, an
external character pointer, loc2, is set as a side effect. The variable
loc2 points to the next character in string after the last character that
matched.
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.
The external variables circf, sed, and nbra are reserved.
DIAGNOSTICS
The function compile uses the macro RETURN on success and the macro ERROR
on failure (see above). The functions step and advance return non-zero
on a successful match and zero if there is no match. Errors are:
11 range endpoint too large.
16 bad number.
25 \ digit out of range.
36 illegal or missing delimiter.
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regexp(5) UNIX System V regexp(5)
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.
EXAMPLE
The following is an example of how the regular expression macros and
calls might be defined by an application program:
#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;
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