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=====discrete Keyword 3=====
combination

====Definition====

A combination is a set containing a certain number of objects that have been selected from another set. In combinations, the order of the elements does not matter.
====References====
List any sites, books, or sources utilized when researching information on this topic. (Remove any filler text).

  * wikipedia
  * class


=====discrete Keyword 3 Phase 2=====
regular expression

====Definition====
Regular expressions are commands that match a pattern or characters in a string of characters or words. Some of the  regular expression commands are the ^ that matches the beginning of a line, the $ matches the end of line, the \< matches the beginning of the line, \> matches the end of the word, . matches any single character, * 0 is 0 or more of the previous character or whatever you put in front of the *, [] matches any of the characters enclosed, [^ ] does not match any of the characters enclosed. Then there is also extended regular characters or egrep. Those are () which is like \( \), | which means or and + which matches 1 or more of the previous character. Usually you have to use a combinations of these to get what you want.

====References====

  * http://en.wikipedia.org/wiki/Regular_expression
  * notes from unix/linux class
====Demonstration====
Demonstration of regular expression.

If you wish to aid your definition with a code sample, you can do so by using a wiki **code** block, an example follows:

<code c>
#include <stdio.h>
#include <regex.h>
int main(void)
{
        regex_t reg;
        const char *regex="[abc]";
        const char *str="sadf";
        regmatch_t matches[16];

        regcomp(&reg, regex, REG_EXTENDED);

        if(regexec(&reg, str, 16, matches, 0) == 0)
        {
                printf("regex /%s/ matched string '%s' at bytes %d-%d\n",
                        regex, str, matches[0].rm_so, matches[0].rm_eo);
        }
        else
                printf("regex /%s/ does not match string '%s'\n", regex, str);
}
</code>

Alternatively (or additionally), if you want to demonstrate something on the command-line, you can do so as follows:

<cli>
lab46:~$ nano test.c
lab46:~$ gcc -otest test.c
lab46:~$ ./test
regex /[abc]/ matched string 'sadf' at bytes 1-2
lab46:~$ 

</cli>
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