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man pages section 3: Extended Library Functions, Volume 2

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Updated: July 2017



regexpr, compile, step, advance - regular expression compile and match routines


cc  [flag]... [file]... -lgen [library]...
#include <regexpr.h>

char *compile(char *instring, char *expbuf, const char *endbuf);
step(const char *string, const char *expbuf);
advance(const char *string, const char *expbuf);
extern char *loc1, loc2, locs;
extern int nbra, regerrno, reglength;
extern char *braslist[], *braelist[];


These routines are used to compile regular expressions and match the compiled expressions against lines. The regular expressions compiled are in the form used by ed(1).

The parameter instring is a null-terminated string representing the regular expression.

The parameter expbuf points to the place where the compiled regular expression is to be placed. If expbuf is NULL, compile() uses malloc(3C) to allocate the space for the compiled regular expression. If an error occurs, this space is freed. It is the user's responsibility to free unneeded space after the compiled regular expression is no longer needed.

The parameter endbuf is one more than the highest address where the compiled regular expression may be placed. This argument is ignored if expbuf is NULL. If the compiled expression cannot fit in (endbufexpbuf) bytes, compile() returns NULL and regerrno (see below) is set to 50.

The parameter string is a pointer to a string of characters to be checked for a match. This string should be null-terminated.

The parameter expbuf is the compiled regular expression obtained by a call of the function compile().

The function 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 variables set in step() are loc1 and loc2. loc1 is a pointer 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 line, loc1 points to the first character of string and loc2 points to the null at the end of string .

The purpose of step() is to step through the string argument until a match is found or until the end of string is reached. If the regular expression begins with ^, step() tries to match the regular expression at the beginning of the string only.

The advance() function is similar to step(); but, it only sets the variable loc2 and always restricts matches to the beginning of the string.

If one is looking for successive matches in the same string of characters, locs should be set equal to loc2, and step() should be called with string equal to loc2. locs is used by commands like ed and sed so that global substitutions like s/y*//g do not loop forever, and is NULL by default.

The external variable nbra is used to determine the number of subexpressions in the compiled regular expression. braslist and braelist are arrays of character pointers that point to the start and end of the nbra subexpressions in the matched string. For example, after calling step() or advance () with string sabcdefg and regular expression \(abcdef\), braslist[0] will point at a and braelist[0] will point at g. These arrays are used by commands like ed and sed for substitute replacement patterns that contain the \ n notation for subexpressions.

Note that it is not necessary to use the external variables regerrno, nbra, loc1, loc2 locs, braelist, and braslist if one is only checking whether or not a string matches a regular expression.


Example 1 The following is similar to the regular expression code from grep:
 . . . 
if(compile(*argv, (char *)0, (char *)0) == (char *)0)
 . . . 
if (step(linebuf, expbuf)) 	
   succeed( );

Return Values

If compile() succeeds, it returns a non-NULL pointer whose value depends on expbuf. If expbuf is non-NULL, compile() returns a pointer to the byte after the last byte in the compiled regular expression. The length of the compiled regular expression is stored in reglength. Otherwise, compile() returns a pointer to the space allocated by malloc(3C).

The functions step() and advance() return non-zero if the given string matches the regular expression, and zero if the expressions do not match.


If an error is detected when compiling the regular expression, a NULL pointer is returned from compile() and regerrno is set to one of the non-zero error numbers indicated below:

Range endpoint too large.
Bad Number.
"\digit" out or range.
Illegal or missing delimiter.
No remembered string search.
\(~\) imbalance.
Too many \(.
More than 2 numbers given in \[~\}.
} expected after \.
First number exceeds second in \{~\}.
[] imbalance.
Regular expression overflow.


See attributes(5) for descriptions of the following attributes:


See Also

ed(1), grep(1), sed(1), malloc(3C), attributes(5), regexp(5)


When compiling multi-threaded applications, the _REENTRANT flag must be defined on the compile line. This flag should only be used in multi-threaded applications.