===== What is a Pattern? =====

The main purpose of regular expressions is to perform advanced pattern matching on text strings.  A pattern defines rules so that a computer can recognize a sequence of characters.

Let's take for example, a pattern which matches a 7-digit phone number.

<code php>
$test = '867-5309';
$pattern = '|^[0-9]{3}-[0-9]{4}$|';
$isphone = preg_match($pattern, $test);
echo 'result = ' . $isphone;
</code>

A pattern like this may seem daunting at first if you are new to regular expressions.  Let's break down the individual components and see how this pattern works.

  * Pipe (**|**) is used to mark the beginning and the end of the pattern.  It basically says that the text between the two pipes is the pattern we want to match.
  * Caret (**^**) means we want to match the beginning of the string.  Without this, the pattern can start matching at any character in the string.  With this, the pattern //CAN ONLY// match at the start of the string.
  * Brackets (**[** and **]**) indicate that we are defining a set or range characters.  By itself, this type of set will only match one character (belonging to the set or range). 
  * Hypen (**-**) when used within brackets indicates that we are defining a range of characters.  So when we say ''[0-9]'', what we mean is any character between 0 and 9 (meaning any digit.)
  * Braces (**{** and **}**) are used to indicate the exact number of occurances that the preceding character should match.  So our digit should match 3 characters in-a-row.
  * Hypen (**-**) when used //OUTSSIDE OF// brackets just matches a normal hyphen character.
  * Dollar Sign (**$**) is used to match the end of the search text.  By including caret (**^**) and dollar sign (**$**), we are telling the pattern matcher that the pattern must match the entire search string.

So the pattern we have made will only match a 7-digit phone number with a hyphen between the first 3 digits and the last 4 digits.  The pattern ''[0-9]'' is such a common pattern, that there exists a special notation to define this (**\d**).  You could also do away with the brace notation and just repeat this sequence the required number of times like:

<code php>
$pattern = '|^\d\d\d-\d\d\d\d$|';
</code>

For simple patterns, this type of pattern is much simpler to understand.  We leave it up to you to decide which is best for your purposes.