Use Regx to quantify a sequence of characters

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"a(bc)*": matches a string that has an a followed by zero or more copies of the sequence "bc";
"a(bc){1,5}": one through five copies of "bc."

There's also the '|' symbol, which works as an OR operator:
"hi|hello": matches a string that has either "hi" or "hello" in it;
"(b|cd)ef": a string that has either "bef" or "cdef";
"(a|b)*c": a string that has a sequence of alternating a's and b's ending in a c;

A period ('.') stands for any single character:
"a.[0-9]": matches a string that has an a followed by one character and a digit;
"^.{3}$": a string with exactly 3 characters.Bracket expressions specify which characters are allowed in a single position of a string:
"[ab]": matches a string that has either an a or a b (that's the same as "a|b");
"[a-d]": a string that has lowercase letters 'a' through 'd' (that's equal to "a|b|c|d" and even "[abcd]");
"^[a-zA-Z]": a string that starts with a letter;
"[0-9]%": a string that has a single digit before a percent sign;
",[a-zA-Z0-9]$": a string that ends in a comma followed by an alphanumeric character.You can also list which characters you DON'T want -- just use a '^' as the first symbol in a bracket expression(i.e., "%[^a-zA-Z]%" matches a string with a character that is not a letter between two percent signs).

In order to be taken literally, you must escape the characters "^.[$()|*+?{\" with a backslash ('\'), asthey have special meaning. On top of that, you must escape the backslash character itself in PHP3 strings, so,for instance, the regular expression "(\$|¥)[0-9]+" would have the function call:

ereg("(\\$|¥)[0-9]+", $str)(what string does that validate?)

Example 1. Examples of valid patterns

1.         * /<\/\w+>/
   
2. 
   
3.         * |(\d{3})-\d+|Sm
   
4. 
   
5.         * /^(?i)php[34]/
   
6. 
   
7.         * {^\s+(\s+)?$}

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Example 2. Examples of invalid patterns

1.         * /href='(.*)' - missing ending delimiter
   
2. 
   
3.         * /\w+\s*\w+/J - unknown modifier 'J'
   
4. 
   
5.         * 1-\d3-\d3-\d4| - missing starting delimiter

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preg_split(PHP 3>= 3.0.9, PHP 4 )preg_split -- Split string by a regular expressionDescriptionarray preg_split ( string pattern, string subject [, int limit [, int flags]])Returns an array containing substrings of subject split along boundaries matched by pattern.If limit is specified, then only substrings up to limit are returned, and if limit is -1, itactually means "no limit", which is useful for specifying the flags.flags can be any combination of the following flags (combined with bitwise | operator)REG_SPLIT_NO_EMPTY    If this flag is set, only non-empty pieces will be returned by preg_split().PREG_SPLIT_DELIM_CAPTURE    If this flag is set, parenthesized expression in the delimiter pattern will be captured and        returned as well. This flag was added for 4.0.5.PREG_SPLIT_OFFSET_CAPTURE    If this flag is set, for every occuring match the appendant string offset will also be        returned. Note that this changes the return value in an array where every element is an        array consisting of the matched string at offset 0 and it's string offset into subject        at offset 1. This flag is available since PHP 4.3.0 .Example 1. preg_split() example : Get the parts of a search string

1. <?php
   
2. // split the phrase by any number of commas or space characters,
   
3. // which include " ", \r, \t, \n and \f
   
4. $keywords = preg_split ("/[\s,]+/", "hypertext language, programming");
   
5. ?>

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Example 2. Splitting a string into component characters

1. <?php
   
2. $str = 'string';
   
3. $chars = preg_split('//', $str, -1, PREG_SPLIT_NO_EMPTY);
   
4. print_r($chars);
   
5. ?>

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Example 3. Splitting a string into matches and their offsets

1. <?php
   
2. $str = 'hypertext language programming';
   
3. $chars = preg_split('/ /', $str, -1, PREG_SPLIT_OFFSET_CAPTURE);
   
4. print_r($chars);
   
5. ?>

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will yield:

Array
(
    [0] => Array
        (
            [0] => hypertext
            [1] => 0
        )

    [1] => Array
        (
            [0] => language
            [1] => 10
        )

    [2] => Array
        (
            [0] => programming
            [1] => 19
        )

)

Note: Parameter flags was added in PHP 4 Beta 3.

preg_match(PHP 3>= 3.0.9, PHP 4 )
preg_match -- Perform a regular expression match
Description
int preg_match ( string pattern, string subject [, array matches [, int flags]])

Searches subject for a match to the regular expression given in pattern.

If matches is provided, then it is filled with the results of search. $matches[0] will
contain the text that matched the full pattern, $matches[1] will have the text that matched
the first captured parenthesized subpattern, and so on.

flags can be the following flag:

PREG_OFFSET_CAPTURE
    If this flag is set, for every occuring match the appendant string offset will also
    be returned. Note that this changes the return value in an array where every element
    is an array consisting of the matched string at offset 0 and it's string offset into
    subject at offset 1. This flag is available since PHP 4.3.0 .

The flags parameter is available since PHP 4.3.0 .

preg_match() returns the number of times pattern matches. That will be either 0 times
    (no match) or 1 time because preg_match() will stop searching after the first match.
preg_match_all() on the contrary will continue until it reaches the end of subject.
preg_match() returns FALSE if an error occured.

    Tip: Do not use preg_match() if you only want to check if one string is contained
    in another string. Use strpos() or strstr() instead as they will be faster.

Example 1. Find the string of text "php"

1. <?php
   
2. // The "i" after the pattern delimiter indicates a case-insensitive search
   
3. if (preg_match ("/php/i", "PHP is the web scripting language of choice.")) {
   
4.     print "A match was found.";
   
5. } else {
   
6.     print "A match was not found.";
   
7. }
   
8. ?>

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Example 2. Find the word "web"

1. <?php
   
2. /* The \b in the pattern indicates a word boundary, so only the distinct
   
3. * word "web" is matched, and not a word partial like "webbing" or "cobweb" */
   
4. if (preg_match ("/\bweb\b/i", "PHP is the web scripting language of choice.")) {
   
5.     print "A match was found.";
   
6. } else {
   
7.     print "A match was not found.";
   
8. }
   
9. 
   
10. if (preg_match ("/\bweb\b/i", "PHP is the website scripting language of choice.")) {
    
11.     print "A match was found.";
    
12. } else {
    
13.     print "A match was not found.";
    
14. }
    
15. ?>

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Example 3. Getting the domain name out of a URL

1. <?php
   
2. // get host name from URL
   
3. preg_match("/^(http:\/\/)?([^\/]+)/i",
   
4.     "http://www.php.net/index.html", $matches);
   
5. $host = $matches[2];
   
6. 
   
7. // get last two segments of host name
   
8. preg_match("/[^\.\/]+\.[^\.\/]+$/", $host, $matches);
   
9. echo "domain name is: {$matches[0]}\n";
   
10. ?>

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This example will produce:domain name is: php.net

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When using Perl-style matching, the pattern also has to be enclosed by special delimiters.
The default is the forward slash, though you can use others. For example:

     /colou?r/

Usually you'll want to stick with the default, but if you need to use the
forward slash a lot in the actual pattern (especially if you're dealing with
pathnames) you might want to use something else:

     !/root/home/random!

To make a match case-insensitive, all you need to do is append the option
i to the pattern:

     /colou?r/i

Perl-style functions support these extra metacharacters (this is not a full
list):

\b A word boundary, the spot between word (\w) and non-word (\W) characters.
     \B A non-word boundary.
     \d A single digit character.
     \D A single non-digit character.
     \n The newline character. (ASCII 10)
     \r The carriage return character. (ASCII 13)
     \s A single whitespace character.
     \S A single non-whitespace character.
     \t The tab character. (ASCII 9)
     \w A single word character - alphanumeric and underscore.
     \W A single non-word character.

Example:

     /\bhomer\b/

Have a donut, Homer no match
     A tale of homeric proportions! no match
     Do you think he can hit a homer? match

Corresponding to ereg() is preg_match(). Syntax:

     preg_match(pattern (string), target (string), optional_array);

Example:

1.      $pattern = "/\b(do(ugh)?nut)\b.*\b(Homer|Fred)\b/i";
   
2. 
   
3. $target = "Have a donut, Homer.";
   
4. 
   
5. if (preg_match($pattern, $target, $matches)) {
   
6. 
   
7. print("<P>Match: $reg[0]</P>");
   
8.      print("<P>Pastry: $reg[1]</P>");
   
9.      print("<P>Variant: $reg[2]</P>");
   
10.      print("<P>Name: $reg[3]</P>");
    
11.      }
    
12. 
    
13. else {
    
14.      print("No match.");
    
15.      }

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Results:

     Match: donut, Homer

Pastry: donut

Variant: [blank because there was no "ugh"]

Name: Homer

     If you use the $target "Doughnut, Frederick?" there will be no match,
     since there has to be a word boundary after Fred.

but "Doughnut, fred?" will match since we've specified it to be
case-insensitive.
   
Contributed code which is applicable (and very useful!)
mkr at binarywerks dot dk
A (AFAIK) correct implementation of Ipv4 validation, this one supports optional ranges
(CIDR notation) and it validates numbers from 0-255 only in the address part, and 1-32
only after the /

1. <?
   
2. 
   
3. function valid_ipv4($ip_addr)
   
4. {
   
5.         $num="([0-9]|1?\d\d|2[0-4]\d|25[0-5])";
   
6.         $range="([1-9]|1\d|2\d|3[0-2])";
   
7. 
   
8.         if(preg_match("/^$num\.$num\.$num\.$num(\/$range)?$/",$ip_addr))
   
9.         {
   
10.                 return 1;
    
11.         }
    
12. 
    
13.         return 0;
    
14. }
    
15. 
    
16. $ip_array[] = "127.0.0.1";
    
17. $ip_array[] = "127.0.0.256";
    
18. $ip_array[] = "127.0.0.1/36";
    
19. $ip_array[] = "127.0.0.1/1";
    
20. 
    
21. foreach ($ip_array as $ip_addr)
    
22. {
    
23.         if(valid_ipv4($ip_addr))
    
24.         {
    
25.                 echo "$ip_addr is valid<BR>\n";
    
26.         }
    
27.         else
    
28.         {
    
29.                 echo "$ip_addr is NOT valid<BR>\n";
    
30.         }
    
31. }
    
32. 
    
33. ?>

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plenque at hotmail dot com
I wrote a function that checks if a given regular expression is valid. I think some of
you might find it useful. It changes the error_handler and restores it, I didn't find
any other way to do it.

Function IsRegExp ($sREGEXP)
{
    $sPREVIOUSHANDLER = Set_Error_Handler ("TrapError");
    Preg_Match ($sREGEXP, "");
    Restore_Error_Handler ($sPREVIOUSHANDLER);
    Return !TrapError ();
}

Function TrapError ()
{
    Static $iERRORES;

    If (!Func_Num_Args ())
    {
        $iRETORNO = $iERRORES;
        $iERRORES = 0;
        Return $iRETORNO;
    }
    Else
    {
        $iERRORES++;
    }
}


PHP Get_title tag code which uses simple regex and nice php string functions
(As from Zend PHP)

<?php
function get_title_tag($chaine){
    $fp = fopen ($chaine, 'r');
    while (! feof ($fp)){
         $contenu .= fgets ($fp, 1024);
         if (stristr($contenu, '<\title>' )){
                 break;
                }
         }
    if (eregi("", $contenu, $out)) {
        return $out[1];
        }
    else{
        return false;
        }
    }
?>

My Own 'Visitor Trac' code which uses regex XML parsing methods

<?php
$referer = $_SERVER['HTTP_REFERER'];
$filename = $_SERVER[REMOTE_ADDR] . '.txt';
//print_r($_SERVER);
if (file_exists($filename)){
    $lastvisit = filectime($filename);
    $currentdate = date('U');
    $difference = round(($currentdate - $lastvisit)/84600);
    if ($difference > 7)  {
        unlink($filename);
        $fp = fopen($filename, "a");
    }
    else $fp = fopen($filename, "a");
}
    else $fp = fopen($filename, "a");
if (!$_SERVER['HTTP_REFERER']) $url_test = 'http://dinki.mine.nu/weblog/';
else $url_test = $_SERVER['HTTP_REFERER'];
$new_title = return_title ($url_test);
//print $new_title;
$new_name = stripslashes("<beg>$new_title\n");
$new_URL = stripslashes("<beg>$referer\n");
fwrite($fp,$new_URL);
fwrite($fp,$new_name);
fclose($fp);

$fp = fopen($filename, "r");
$file = implode('', file ($filename));
$foo = preg_split("/<beg>/",$file);
$number = count($foo);
//print $number;
if ($number > 11) {
    fclose($fp);
    $fp = fopen($filename, "w");
    $count = $number - 10;
    while ($count < $number)  {
        $print1 = $foo[$count];
        $print2 = $foo[$count+1];
        print " <img src = arrow.gif> ";
        print "<a href=$print1>$print2</a>"; //print $count;
        $count += 2;
        $new_name = stripslashes("<beg>$print2");
        $new_URL = stripslashes("<beg>$print1");
        fwrite($fp,$new_URL);
        fwrite($fp,$new_name);
    }
    fclose($fp);
}
//print_r($foo);
else  {
    $count = 1;
    while ($count <= $number)  {
        $print1 = $foo[$count];
        $print2 = $foo[$count+1];
        print " <img src = arrow.gif> ";
        print "<a href=$print1>$print2</a>"; //print $count;
        $count += 2;
        }
    fclose($fp);
    }

function return_title($url)  {
    print $filename." ".$difference;
         $array = file ($url);
          for ($i = 0; $i < count($array); $i++)
          {
            if (preg_match("/<title>(.*)<\/title>/i",$array[$i], $tag_contents))  {
                $title = $tag_contents[1];
                $title = strip_tags($title);
                }
          }
          return $title;
    }

?>