$pattern = "^[a-zA-Z0-9_]+@[a-zA-Z0-9_]+(\\.[a-zA-Z0-9_]+)+";
if( ereg($pattern, $email) ){
    echo "E-mail address is valid";
}
else{
    echo "Invalid e-mail address";
}

$pattern = "^([a-zA-Z0-9_])+@([a-zA-Z0-9_]+)\\.([a-zA-Z0-9_]+)$";
if( ereg($pattern, $email, $arr) ){
    echo "E-mail address is valid";
    for($i=0;$arr[$i];$i++)
        echo "\$arr[$i] = $arr[$i]";
}
else{
    echo "Invalid e-mail address";
}

Regular expression replace

$card = "1234-2345-5677-5675";
$pattern = "[0-9]{4}";
$Card = ereg_replace($pattern, "****", $card);
echo "$card";        // stays the same
echo "$Card";        // now equals ****-****-****-****

The complete documentation can be found on the PHP site. Please also use this page to play with different regular expressions and PHP functions.

Perl-style regular expressions in PHP.

Search for a match.

int preg_match(pattern, string [, matcharray]);
int preg_match_all(pattern, string, matcharray[, flag]);

$pregexep = "/^(\d{3}) \d{3}-\d{4}$/";

$preg = "/\bweb\b/i";
$str = array( "PHP is the Web scripting language of choice.",
              "PHP is the website scripting language of choice.");
for($i=0;$i<count($str);$i++)			  
  if( preg_match($preg, $str[$i]) )
    print "String: '$str[$i]' : a match was found.<br>";
  else
    print "String: '$str[$i]' : a match was not found.<br>";

If the third (optional) argument is provided it will be filled with the result of the search. The first element of this array will contain the substring of the subject string that matches the pattern, the second argument will contain the substring of the match that matches the first sub-pattern in the pattern (a pattern inside the first parenthesis), and so on. Please notice that preg_match() stops searching after the first match is found. To find all matches in a string we need to use function preg_match_all().

This functions takes the same arguments function preg_match() does, but the result array is a two-dimensional array. This array contains all substrings of the subject string that match the pattern as well as all sub-pattern matches. How exactly these elements are stored in the array depends on the value of the last (optional) parameter flag. We would prefer use value PREG_SET_ORDER for this argument. In this case each element of the result array is exactly an array returned by function preg_match(). The following example illustrates how to extract some opening and closing tags from an HTML file and also text between them:

$html = "<a href='http://www.php.net'>PHP site</a> <b>please</b> go there";
$preg = "/<([\w]+)( [^>]*)?>(.*)<\/\\1>/";
if( preg_match_all($preg, $html, $res, PREG_SET_ORDER) )
  for($i=0;$i<count($res);$i++){
    print "<ul>Tag $i:";
    for($j=0;$j<count($res[$i]);$j++)
      print "<li> " . preg_replace("<", "&lt;", $res[$i][$j]);
    print "</ul>";
  }
else
  print "There is no math";

Regular expression replacement.

string preg_replace(pattern, replacement, string);

replacement argument may contain references of the form \\n or (since PHP 4.0.4) $n, with the latter form being the preferred one. Every such reference will be replaced by the text captured by the n'th parenthesized pattern. n can be from 0 to 99, and \\0 or $0 refers to the text matched by the whole pattern. Opening parentheses are counted from left to right (starting from 1) to obtain the number of the capturing subpattern.

Using a regular expression as a delimiter.

array preg_split(pattern, string);

$sentence = "For example, we may want to get all words used in a sentence, ".
            "but we do not know for sure how    these words are separated, ".
            "with\n spaces (how many?), tabs, \t commas, new lines, etc.";
$delimiter = "/[\s,]+/";
$words = preg_split($delimiter, $sentence);
for($i=0;$i<count($words);$i++)
   print "<li> $words[$i]";

Note: some of the regular expression functions have more options than described in this lecture. Please consult PHP documentation about the complete description. The following Perl-style regular expression play ground to use your own regular expressions.

Some string functions.