On-line Manual Page for 5(regexp)
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compile(), step(), advance() - regular expression compile and match routines


#define INIT declarations
#define GETC() getc statements
#define PEEKC() peekc statements
#define UNGETC(c) ungetc statements
#define RETURN(pointer) return statements
#define ERROR(val) error statements

#include <regexp.h>

char *compile(
const char *instring,
char *expbuf,
const char *endbuf,
int eof

int step(const char *string, const char *expbuf);

int advance(const char *string, const char *expbuf);

extern char *loc1, *loc2, *locs;

extern int circf, sed, nbra;

Features documented in this manual entry are obsolescent and may be removed in a future HP-UX release. Use of regcomp(3C) functions instead is recommended.


These functions are general-purpose regular expression matching routines to be used in programs that perform Basic Regular Expression (see regexp(5)) matching. These functions are defined in <regexp.h>.

The functions step() and advance() do pattern matching given a character string and a compiled regular expression as input. compile() takes a Basic Regular Expression as input and produces a compiled expression that can be used with step() and advance().

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.

Return the value of the next byte in the regular expression pattern. Successive calls to GETC() should return successive bytes of the regular expression.

Return the next byte in the regular expression. Successive calls to PEEKC() should return the same byte (which should also be the next byte returned by GETC().

Cause the argument c to be returned by the next call to GETC() (and PEEKC()). No more than one byte of pushback is ever needed, and this byte is guaranteed to be the last byte read by GETC(). The value of the macro UNGETC(c) is always ignored.

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 that must manage memory allocation.

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.
Range endpoint too large.
Bad number.
``\digit'' out of range.
Illegal or missing delimiter.
No remembered search string.
\( \) imbalance.
Too many \(.
More than 2 numbers given in \{ \}.
} expected after \.
First number exceeds second in \{ \}.
[ ] imbalance.
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 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 location where the compiled regular expression will be placed.

The parameter endbuf is one more than the highest address where the compiled regular expression can 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 is 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 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 the example below of the declarations taken from grep(1).

step() also performs actual regular expression matching in this file. 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 that was obtained by a call to compile().

step() returns non-zero 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 points to the first character of string and loc2 points 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, step() tries 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().

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 non-zero, which indicates a match, or until the end of string is reached. To constrain string to beginning-of-line in all cases, step() need not be called; simply call advance().

When advance() encounters a * or \{\} sequence in the regular expression, it advances its pointer to the string to be matched as far as possible and recursively calls itself, trying to match the rest of the string to the rest of the regular expression. As long as there is no match, advance backs 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() breaks out of the loop that backs up and returns 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 such as s/y*//g do not loop forever.

The additional external variables sed and nbra are used for special purposes.


The LC_COLLATE category determines the collating sequence used in compiling and executing regular expressions.

The LC_CTYPE category determines the interpretation of text as single and/or multi-byte characters, and the characters matched by character class expressions in regular expressions.

International Code Set Support
Single- and multi-byte character code sets are supported.


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()
#define PEEKC()
#define UNGETC(c)
#define RETURN(c)
#define ERROR(c)

#include <regexp.h>
(void) compile(*argv, expbuf, &expbuf[ESIZE], `\0'); ...
if (step(linebuf, expbuf))


regexp() was developed by OSF and HP.


grep(1), regcomp(3C), setlocale(3C), regexp(5).


advance(): AES, SVID2, SVID3, XPG2, XPG3, XPG4

compile(): AES, SVID2, SVID3, XPG2, XPG3, XPG4

loc1: AES, SVID2, SVID3, XPG2, XPG3, XPG4

loc2: AES, SVID2, SVID3, XPG2, XPG3, XPG4

locs: AES, SVID2, SVID3, XPG2, XPG3, XPG4

step(): AES, SVID2, SVID3, XPG2, XPG3, XPG4

But what do you want from section 5?

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