FreeBSD Manual Pages

home | help
PCRE2DEMO(3)		   Library Functions Manual		  PCRE2DEMO(3)

NAME
       PCRE2DEMO - A demonstration C program for PCRE2

SOURCE CODE

/*************************************************
*	    PCRE2 DEMONSTRATION	PROGRAM		 *
*************************************************/

/* This	is a demonstration program to illustrate a straightforward way of
using the PCRE2	regular	expression library from	a C program. See the
pcre2sample documentation for a	short discussion ("man pcre2sample" if you have
the PCRE2 man pages installed).	PCRE2 is a revised API for the library,	and is
incompatible with the original PCRE API.

There are actually three libraries, each supporting a different	code unit
width. This demonstration program uses the 8-bit library. The default is to
process	each code unit as a separate character,	but if the pattern begins with
"(*UTF)", both it and the subject are treated as UTF-8 strings,	where
characters may occupy multiple code units.

In Unix-like environments, if PCRE2 is installed in your standard system
libraries, you should be able to compile this program using this command:

cc -Wall pcre2demo.c -lpcre2-8 -o pcre2demo

If PCRE2 is not	installed in a standard	place, it is likely to be installed
with support for the pkg-config	mechanism. If you have pkg-config, you can
compile	this program using this	command:

cc -Wall pcre2demo.c `pkg-config --cflags --libs libpcre2-8` -o	pcre2demo

If you do not have pkg-config, you may have to use something like this:

cc -Wall pcre2demo.c -I/usr/local/include -L/usr/local/lib \
  -R/usr/local/lib -lpcre2-8 -o	pcre2demo

Replace	"/usr/local/include" and "/usr/local/lib" with wherever	the include and
library	files for PCRE2	are installed on your system. Only some	operating
systems	(Solaris is one) use the -R option.

Building under Windows:

If you want to statically link this program against a non-dll .a file, you must
define PCRE2_STATIC before including pcre2.h, so in this environment, uncomment
the following line. */

/* #define PCRE2_STATIC	*/

/* The PCRE2_CODE_UNIT_WIDTH macro must	be defined before including pcre2.h.
For a program that uses	only one code unit width, setting it to	8, 16, or 32
makes it possible to use generic function names	such as	pcre2_compile(). Note
that just changing 8 to	16 (for	example) is not	sufficient to convert this
program	to process 16-bit characters. Even in a	fully 16-bit environment, where
string-handling	functions such as strcmp() and printf()	work with 16-bit
characters, the	code for handling the table of named substrings	will still need
to be modified.	*/

#define	PCRE2_CODE_UNIT_WIDTH 8

#include <stdio.h>
#include <string.h>
#include <pcre2.h>

/**************************************************************************
* Here is the program. The API includes	the concept of "contexts" for	  *
* setting up unusual interface requirements for	compiling and matching,	  *
* such as custom memory	managers and non-standard newline definitions.	  *
* This program does not	do any of this,	so it makes no use of contexts,	  *
* always passing NULL where a context could be given.			  *
**************************************************************************/

int main(int argc, char	**argv)
{
pcre2_code *re;
PCRE2_SPTR pattern;	/* PCRE2_SPTR is a pointer to unsigned code units of */
PCRE2_SPTR subject;	/* the appropriate width (in this case,	8 bits). */
PCRE2_SPTR name_table;

int crlf_is_newline;
int errornumber;
int find_all;
int i;
int rc;
int utf8;

uint32_t option_bits;
uint32_t namecount;
uint32_t name_entry_size;
uint32_t newline;

PCRE2_SIZE erroroffset;
PCRE2_SIZE *ovector;
PCRE2_SIZE subject_length;

pcre2_match_data *match_data;

/**************************************************************************
* First, sort out the command line. There is only one possible option at  *
* the moment, "-g" to request repeated matching	to find	all occurrences,  *
* like Perl's /g option. We set	the variable find_all to a non-zero value *
* if the -g option is present.						  *
**************************************************************************/

find_all = 0;
for (i = 1; i <	argc; i++)
  {
  if (strcmp(argv[i], "-g") == 0) find_all = 1;
  else if (argv[i][0] == '-')
    {
    printf("Unrecognised option	%s\n", argv[i]);
    return 1;
    }
  else break;
  }

/* After the options, we require exactly two arguments,	which are the pattern,
and the	subject	string.	*/

if (argc - i !=	2)
  {
  printf("Exactly two arguments	required: a regex and a	subject	string\n");
  return 1;
  }

/* Pattern and subject are char	arguments, so they can be straightforwardly
cast to	PCRE2_SPTR because we are working in 8-bit code	units. The subject
length is cast to PCRE2_SIZE for completeness, though PCRE2_SIZE is in fact
defined	to be size_t. */

pattern	= (PCRE2_SPTR)argv[i];
subject	= (PCRE2_SPTR)argv[i+1];
subject_length = (PCRE2_SIZE)strlen((char *)subject);

/*************************************************************************
* Now we are going to compile the regular expression pattern, and handle *
* any errors that are detected.						 *
*************************************************************************/

re = pcre2_compile(
  pattern,		 /* the	pattern	*/
  PCRE2_ZERO_TERMINATED, /* indicates pattern is zero-terminated */
  0,			 /* default options */
  &errornumber,		 /* for	error number */
  &erroroffset,		 /* for	error offset */
  NULL);		 /* use	default	compile	context	*/

/* Compilation failed: print the error message and exit. */

if (re == NULL)
  {
  PCRE2_UCHAR buffer[256];
  pcre2_get_error_message(errornumber, buffer, sizeof(buffer));
  printf("PCRE2	compilation failed at offset %d: %s\n",	(int)erroroffset,
    buffer);
  return 1;
  }

/*************************************************************************
* If the compilation succeeded,	we call	PCRE2 again, in	order to do a	 *
* pattern match	against	the subject string. This does just ONE match. If *
* further matching is needed, it will be done below. Before running the	 *
* match	we must	set up a match_data block for holding the result. Using	 *
* pcre2_match_data_create_from_pattern() ensures that the block	is	 *
* exactly the right size for the number	of capturing parentheses in the	 *
* pattern. If you need to know the actual size of a match_data block as	 *
* a number of bytes, you can find it like this:				 *
*									 *
* PCRE2_SIZE match_data_size = pcre2_get_match_data_size(match_data);	 *
*************************************************************************/

match_data = pcre2_match_data_create_from_pattern(re, NULL);

/* Now run the match. */

rc = pcre2_match(
  re,			/* the compiled	pattern	*/
  subject,		/* the subject string */
  subject_length,	/* the length of the subject */
  0,			/* start at offset 0 in	the subject */
  0,			/* default options */
  match_data,		/* block for storing the result	*/
  NULL);		/* use default match context */

/* Matching failed: handle error cases */

if (rc < 0)
  {
  switch(rc)
    {
    case PCRE2_ERROR_NOMATCH: printf("No match\n"); break;
    /*
    Handle other special cases if you like
    */
    default: printf("Matching error %d\n", rc);	break;
    }
  pcre2_match_data_free(match_data);   /* Release memory used for the match */
  pcre2_code_free(re);		       /*   data and the compiled pattern. */
  return 1;
  }

/* Match succeeded. Get	a pointer to the output	vector,	where string offsets
are stored. */

ovector	= pcre2_get_ovector_pointer(match_data);
printf("Match succeeded	at offset %d\n", (int)ovector[0]);

/*************************************************************************
* We have found	the first match	within the subject string. If the output *
* vector wasn't	big enough, say	so. Then output	any substrings that were *
* captured.								 *
*************************************************************************/

/* The output vector wasn't big	enough.	This should not	happen,	because	we used
pcre2_match_data_create_from_pattern() above. */

if (rc == 0)
  printf("ovector was not big enough for all the captured substrings\n");

/* Since release 10.38 PCRE2 has locked	out the	use of \K in lookaround
assertions. However, there is an option	to re-enable the old behaviour.	If that
is set,	it is possible to run patterns such as /(?=.\K)/ that use \K in	an
assertion to set the start of a	match later than its end. In this demonstration
program, we show how to	detect this case, but it shouldn't arise because the
option is never	set. */

if (ovector[0] > ovector[1])
  {
  printf("\\K was used in an assertion to set the match	start after its	end.\n"
    "From end to start the match was: %.*s\n", (int)(ovector[0]	- ovector[1]),
      (char *)(subject + ovector[1]));
  printf("Run abandoned\n");
  pcre2_match_data_free(match_data);
  pcre2_code_free(re);
  return 1;
  }

/* Show	substrings stored in the output	vector by number. Obviously, in	a real
application you	might want to do things	other than print them. */

for (i = 0; i <	rc; i++)
  {
  PCRE2_SPTR substring_start = subject + ovector[2*i];
  PCRE2_SIZE substring_length =	ovector[2*i+1] - ovector[2*i];
  printf("%2d: %.*s\n",	i, (int)substring_length, (char	*)substring_start);
  }

/**************************************************************************
* That concludes the basic part	of this	demonstration program. We have	  *
* compiled a pattern, and performed a single match. The	code that follows *
* shows	first how to access named substrings, and then how to code for	  *
* repeated matches on the same subject.					  *
**************************************************************************/

/* See if there	are any	named substrings, and if so, show them by name.	First
we have	to extract the count of	named parentheses from the pattern. */

(void)pcre2_pattern_info(
  re,			/* the compiled	pattern	*/
  PCRE2_INFO_NAMECOUNT,	/* get the number of named substrings */
  &namecount);		/* where to put	the answer */

if (namecount == 0) printf("No named substrings\n"); else
  {
  PCRE2_SPTR tabptr;
  printf("Named	substrings\n");

  /* Before we can access the substrings, we must extract the table for
  translating names to numbers,	and the	size of	each entry in the table. */

  (void)pcre2_pattern_info(
    re,			      /* the compiled pattern */
    PCRE2_INFO_NAMETABLE,     /* address of the	table */
    &name_table);	      /* where to put the answer */

  (void)pcre2_pattern_info(
    re,			      /* the compiled pattern */
    PCRE2_INFO_NAMEENTRYSIZE, /* size of each entry in the table */
    &name_entry_size);	      /* where to put the answer */

  /* Now we can	scan the table and, for	each entry, print the number, the name,
  and the substring itself. In the 8-bit library the number is held in two
  bytes, most significant first. */

  tabptr = name_table;
  for (i = 0; i	< namecount; i++)
    {
    int	n = (tabptr[0] << 8) | tabptr[1];
    printf("(%d) %*s: %.*s\n", n, name_entry_size - 3, tabptr +	2,
      (int)(ovector[2*n+1] - ovector[2*n]), subject + ovector[2*n]);
    tabptr += name_entry_size;
    }
  }

/*************************************************************************
* If the "-g" option was given on the command line, we want to continue	 *
* to search for	additional matches in the subject string, in a similar	 *
* way to the /g	option in Perl.	This turns out to be trickier than you	 *
* might	think because of the possibility of matching an	empty string.	 *
* What happens is as follows:						 *
*									 *
* If the previous match	was NOT	for an empty string, we	can just start	 *
* the next match at the	end of the previous one.			 *
*									 *
* If the previous match	WAS for	an empty string, we can't do that, as it *
* would	lead to	an infinite loop. Instead, a call of pcre2_match() is	 *
* made with the	PCRE2_NOTEMPTY_ATSTART and PCRE2_ANCHORED flags	set. The *
* first	of these tells PCRE2 that an empty string at the start of the	 *
* subject is not a valid match;	other possibilities must be tried. The	 *
* second flag restricts	PCRE2 to one match attempt at the initial string *
* position. If this match succeeds, an alternative to the empty	string	 *
* match	has been found,	and we can print it and	proceed	round the loop,	 *
* advancing by the length of whatever was found. If this match does not	 *
* succeed, we still stay in the	loop, advancing	by just	one character.	 *
* In UTF-8 mode, which can be set by (*UTF) in the pattern, this may be	 *
* more than one	byte.							 *
*									 *
* However, there is a complication concerned with newlines. When the	 *
* newline convention is	such that CRLF is a valid newline, we must	 *
* advance by two characters rather than	one. The newline convention can	 *
* be set in the	regex by (*CR),	etc.; if not, we must find the default.	 *
*************************************************************************/

if (!find_all)	   /* Check for	-g */
  {
  pcre2_match_data_free(match_data);  /* Release the memory that was used */
  pcre2_code_free(re);		      /* for the match data and	the pattern. */
  return 0;			      /* Exit the program. */
  }

/* Before running the loop, check for UTF-8 and	whether	CRLF is	a valid	newline
sequence. First, find the options with which the regex was compiled and	extract
the UTF	state. */

(void)pcre2_pattern_info(re, PCRE2_INFO_ALLOPTIONS, &option_bits);
utf8 = (option_bits & PCRE2_UTF) != 0;

/* Now find the	newline	convention and see whether CRLF	is a valid newline
sequence. */

(void)pcre2_pattern_info(re, PCRE2_INFO_NEWLINE, &newline);
crlf_is_newline	= newline == PCRE2_NEWLINE_ANY ||
		  newline == PCRE2_NEWLINE_CRLF	||
		  newline == PCRE2_NEWLINE_ANYCRLF;

/* Loop	for second and subsequent matches */

for (;;)
  {
  uint32_t options = 0;			  /* Normally no options */
  PCRE2_SIZE start_offset = ovector[1];	  /* Start at end of previous match */

  /* If	the previous match was for an empty string, we are finished if we are
  at the end of	the subject. Otherwise,	arrange	to run another match at	the
  same point to	see if a non-empty match can be	found. */

  if (ovector[0] == ovector[1])
    {
    if (ovector[0] == subject_length) break;
    options = PCRE2_NOTEMPTY_ATSTART | PCRE2_ANCHORED;
    }

  /* If	the previous match was not an empty string, there is one tricky	case to
  consider. If a pattern contains \K within a lookbehind assertion at the
  start, the end of the	matched	string can be at the offset where the match
  started. Without special action, this	leads to a loop	that keeps on matching
  the same substring. We must detect this case and arrange to move the start on
  by one character. The	pcre2_get_startchar() function returns the starting
  offset that was passed to pcre2_match(). */

  else
    {
    PCRE2_SIZE startchar = pcre2_get_startchar(match_data);
    if (start_offset <=	startchar)
      {
      if (startchar >= subject_length) break;	/* Reached end of subject.   */
      start_offset = startchar + 1;		/* Advance by one character. */
      if (utf8)					/* If UTF-8, it	may be more  */
	{					/*   than one code unit.     */
	for (; start_offset < subject_length; start_offset++)
	  if ((subject[start_offset] & 0xc0) !=	0x80) break;
	}
      }
    }

  /* Run the next matching operation */

  rc = pcre2_match(
    re,			  /* the compiled pattern */
    subject,		  /* the subject string	*/
    subject_length,	  /* the length	of the subject */
    start_offset,	  /* starting offset in	the subject */
    options,		  /* options */
    match_data,		  /* block for storing the result */
    NULL);		  /* use default match context */

  /* This time,	a result of NOMATCH isn't an error. If the value in "options"
  is zero, it just means we have found all possible matches, so	the loop ends.
  Otherwise, it	means we have failed to	find a non-empty-string	match at a
  point	where there was	a previous empty-string	match. In this case, we	do what
  Perl does: advance the matching position by one character, and continue. We
  do this by setting the "end of previous match" offset, because that is picked
  up at	the top	of the loop as the point at which to start again.

  There	are two	complications: (a) When	CRLF is	a valid	newline	sequence, and
  the current position is just before it, advance by an	extra byte. (b)
  Otherwise we must ensure that	we skip	an entire UTF character	if we are in
  UTF mode. */

  if (rc == PCRE2_ERROR_NOMATCH)
    {
    if (options	== 0) break;			/* All matches found */
    ovector[1] = start_offset +	1;		/* Advance one code unit */
    if (crlf_is_newline	&&			/* If CRLF is a	newline	& */
	start_offset < subject_length -	1 &&	/* we are at CRLF, */
	subject[start_offset] == '\r' &&
	subject[start_offset + 1] == '\n')
      ovector[1] += 1;				/* Advance by one more.	*/
    else if (utf8)				/* Otherwise, ensure we	*/
      {						/* advance a whole UTF-8 */
      while (ovector[1]	< subject_length)	/* character. */
	{
	if ((subject[ovector[1]] & 0xc0) != 0x80) break;
	ovector[1] += 1;
	}
      }
    continue;	 /* Go round the loop again */
    }

  /* Other matching errors are not recoverable.	*/

  if (rc < 0)
    {
    printf("Matching error %d\n", rc);
    pcre2_match_data_free(match_data);
    pcre2_code_free(re);
    return 1;
    }

  /* Match succeeded */

  printf("\nMatch succeeded again at offset %d\n", (int)ovector[0]);

  /* The match succeeded, but the output vector	wasn't big enough. This
  should not happen. */

  if (rc == 0)
    printf("ovector was	not big	enough for all the captured substrings\n");

  /* We	must guard against patterns such as /(?=.\K)/ that use \K in an
  assertion to set the start of	a match	later than its end. In this
  demonstration	program, we just detect	this case and give up. */

  if (ovector[0] > ovector[1])
    {
    printf("\\K	was used in an assertion to set	the match start	after its end.\n"
      "From end	to start the match was:	%.*s\n", (int)(ovector[0] - ovector[1]),
	(char *)(subject + ovector[1]));
    printf("Run	abandoned\n");
    pcre2_match_data_free(match_data);
    pcre2_code_free(re);
    return 1;
    }

  /* As	before,	show substrings	stored in the output vector by number, and then
  also any named substrings. */

  for (i = 0; i	< rc; i++)
    {
    PCRE2_SPTR substring_start = subject + ovector[2*i];
    size_t substring_length = ovector[2*i+1] - ovector[2*i];
    printf("%2d: %.*s\n", i, (int)substring_length, (char *)substring_start);
    }

  if (namecount	== 0) printf("No named substrings\n"); else
    {
    PCRE2_SPTR tabptr =	name_table;
    printf("Named substrings\n");
    for	(i = 0;	i < namecount; i++)
      {
      int n = (tabptr[0] << 8) | tabptr[1];
      printf("(%d) %*s:	%.*s\n", n, name_entry_size - 3, tabptr	+ 2,
	(int)(ovector[2*n+1] - ovector[2*n]), subject +	ovector[2*n]);
      tabptr +=	name_entry_size;
      }
    }
  }	 /* End	of loop	to find	second and subsequent matches */

printf("\n");
pcre2_match_data_free(match_data);
pcre2_code_free(re);
return 0;
}

/* End of pcre2demo.c */

PCRE2 10.45			31 August 2021			  PCRE2DEMO(3)
NAME | SOURCE CODE
Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=pcre2demo&sektion=3&manpath=FreeBSD+Ports+14.3.quarterly>
home | help
Header And Logo

Peripheral Links

Site Navigation

FreeBSD Manual Pages

Header And Logo

Peripheral Links

Search

Site Navigation

FreeBSD Manual Pages
