# pcre2demo man page

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## 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 errornumber;
    int find_all, caseless_match;
    int i;
    int rc;

    uint32_t namecount;
    uint32_t name_entry_size;

    PCRE2_SIZE erroroffset;
    PCRE2_SIZE *ovector;
    PCRE2_SIZE ovector_last[2];
    PCRE2_SIZE subject_length;

    pcre2_match_data *match_data;

    /**************************************************************************
    * First, sort out the command line. Options:                              *
    * - "-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.                                    *
    * - "-i" to request caseless matching, like Perl's /i option.  We set the *
    *   variable caseless_match to PCRE2_CASELESS if the -i option is         *
    *   present.                                                              *
    **************************************************************************/

    find_all = 0;
    caseless_match = 0;
    for (i = 1; i < argc; i++)
      {
      if (strcmp(argv[i], "-g") == 0) find_all = 1;
      else if (strcmp(argv[i], "-i") == 0) caseless_match = PCRE2_CASELESS;
      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 */
      caseless_match,        /* possibly enable caseless */
      &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. This is the recommended behaviour. However, the option
    PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK allows applications 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, although it cannot 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.    *
    *                                                                        *
    * To help with this task, PCRE2 provides the pcre2_next_match() helper.  *
    *************************************************************************/

    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. */
      }

    /* Loop for second and subsequent matches */

    ovector_last[0] = ovector[0];
    ovector_last[1] = ovector[1];

    for (;;)
      {
      PCRE2_SIZE start_offset;
      uint32_t options;

      /* After each successful match, we use pcre2_next_match() to obtain the match
      parameters for subsequent match attempts. */

      if (!pcre2_next_match(match_data, &start_offset, &options))
        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 */

      /* If this match attempt fails, exit the loop for subsequent matches. */

      if (rc == PCRE2_ERROR_NOMATCH)
        break;

      /* 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;
        }

      /* This demonstration program depends on pcre2_next_match() to ensure that the
      loop for second and subsequent matches does not run forever. However, it would
      be robust practice for a production application to verify this. The following
      block of code shows how to do this. This error case is not reachable unless
      there is a bug in PCRE2.

      Because this program does not set the PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK option,
      the logic is simple. We verify that either ovector[1] has advanced, or that we
      have an empty match touching the end of a previous non-empty match. See the
      API documentation for guidance if your application uses
      PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK and searches for multiple matches. */

      if (!(ovector[1] > ovector_last[1] ||
            (ovector[1] == ovector[0] && ovector_last[1] > ovector_last[0] &&
             ovector[1] == ovector_last[1])))
        {
        printf("\\K was used in an assertion to yield non-advancing matches.\n");
        printf("Run abandoned\n");
        pcre2_match_data_free(match_data);
        pcre2_code_free(re);
        return 1;
        }

      ovector_last[0] = ovector[0];
      ovector_last[1] = ovector[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 guard against patterns such as /(?=.\K)/ that use \K in an assertion to
      set the start of a match later than its end. As explained above, this case
      should not occur because this demonstration program does not set the
      PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK option, however, we do include code showing
      how to detect it. */

      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 */

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