regcomp Subroutine

Purpose

Compiles a specified basic or extended regular expression into an executable string.

Library

Standard C Library (libc. a)

Syntax

#include <regex.h>

int regcomp ( Preg, 
Pattern,  CFlags)

const char *Preg;
const char *Pattern;
int CFlags;

Description

The regcomp subroutine compiles the basic or extended regular expression that is specified by the Pattern parameter and places the output in the structure pointed to by the Preg parameter.

Parameters

Table 1. Parameters
Item	Description
`Preg`	Specifies the structure to receive the compiled output of the regcomp subroutine.
`Pattern`	Contains the basic or extended regular expression to be compiled by the regcomp subroutine. The default regular expression type for the `Pattern` parameter is a basic regular expression. An application can specify extended regular expressions with the `REG_EXTENDED` flag. The maximum number of subexpressions in an extended regular expression is 23.
`CFlags`	Contains the bitwise inclusive OR of 0 or more flags for the regcomp subroutine. These flags are defined in the regex.h file: REG_EXTENDED Uses extended regular expressions. The maximum number of subexpressions in an extended regular expression is 23. REG_ICASE Ignores case in match. REG_NOSUB Reports only success or failure in the regexec subroutine. If this flag is not set, the regcomp subroutine sets the re_nsub structure to the number of parenthetic expressions found in the `Pattern` parameter. REG_NEWLINE Prohibits. (Period) and nonmatching bracket expression from matching a new-line character. The `^` (circumflex) and $ (dollar sign) should match the zero-length string immediately following or preceding a new-line character.

Return Values

If successful, the regcomp subroutine returns a value of 0. Otherwise, it returns another value indicating the type of failure, and the content of the Preg parameter is undefined.

Error Codes

The following macro names for error codes can be written to the errno global variable under error conditions:

Table 2. Error Codes
Item	Description
`REG_BADPAT`	Indicates a basic or extended regular expression that is not valid.
`REG_ECOLLATE`	Indicates a collating element referenced that is not valid.
`REG_ECTYPE`	Indicates a character class-type reference that is not valid.
`REG_EESCAPE`	Indicates a trailing `\` in pattern.
`REG_ESUBREG`	Indicates a number in `\digit` is not valid or in error.
`REG_EBRACK`	Indicates a `[]` imbalance.
`REG_EPAREN`	Indicates a `` or `()` imbalance.
`REG_EBRACE`	Indicates a `\{\}` imbalance.
`REG_BADBR`	Indicates the content of `\{\}` is unusable: not a number, number too large, more than two numbers, or first number larger than second.
`REG_ERANGE`	Indicates an unusable end point in a range expression.
`REG_ESPACE`	Indicates out of memory.
`REG_BADRPT`	Indicates a? (question mark), * (asterisk), or + (plus sign) not preceded by valid basic or extended regular expression.

If the regcomp subroutine detects an illegal basic or extended regular expression, it can return either the REG_BADPAT error code or another that more precisely describes the error.

Examples

The following example illustrates how to match a string (specified in the string parameter) against an extended regular expression (specified in the Pattern parameter):

#include <sys/types.h>
#include <regex.h>
int
match(char *string, char *pattern)
{
   int     status;
   regex_t re;

   if (regcomp(&re, pattern, REG_EXTENDED|REG_NOSUB) != 0)  {
           return(0) ;              /* report error */
   }
   status = regexec(&re, string, (size_t) 0, NULL, 0);
   regfree(&re);
   if (status != 0)  {
           return(0) ;              /* report error */
   }
   return(1);
}

In the preceding example, errors are treated as no match. When there is no match or error, the calling process can get details by calling the regerror subroutine.