PHP Regular Expression Examples

The main use of regular expressions (also called regex or regexp) is to search for patterns in a given string.

These search patterns are written using a special syntax which a regular expression parser understands. For many developers, Regular Expression seems to be hard to learn and use. But it's not as hard as we think.

So before going into details, here are some useful and reusable PHP Regular Expression codes:

$value = preg_replace("/[^A-Za-z0-9]/","",$value);

Validate username which consist

• alpha-numeric (a-z, A-Z, 0-9)
• underscores
• minimum 5 character and maximum 20 character

You can change the minimum and maximum character to any number.

$username = "user_name12"; 
if (preg_match('/^[a-z\d_]{5,20}$/i', $username)) { 
    echo "Your username is ok."; 
} else { 
    echo "Wrong username format."; 
}

Creates a YouTube thumbnail URL from a YouTube video URL.

function youtube_video_thumbnail($url,$index=0){ 
    if(preg_match('/^[^v]+v.(.{11}).*/',$url,$matches)){ 
        return 'http://img.youtube.com/vi/'.$matches[1].'/'.$index.'.jpg'; 
    }elseif(preg_match('/youtube.com\/user\/(.*)\/(.*)$/',$url,$matches)){ 
        return 'http://img.youtube.com/vi/'.$matches[2].'/'.$index.'.jpg'; 
    }else{ 
        return false; 
    } 
}

preg_replace("/http:\/\/([^\/]+)[^\s]*/", "$1", $text);

This is a useful PHP function. If an image in your content is missing the alt tag, it adds that posts title as the alt tag for the image.

function add_alt_tags($content) 
{ 
    global $post; 
    preg_match_all('/<img (.*?)\/ >/', $content, $images); 
    if(!is_null($images)) 
    { 
        foreach($images[1] as $index => $value) 
        { 
            if(!preg_match('/alt=/', $value)) 
            { 
                $new_img = str_replace('<img', '<img alt="'.get_the_title().'"', $images[0][$index]); 
                $content = str_replace($images[0][$index], $new_img, $content); 
            } 
        } 
    } 
    return $content; 
} 
add_filter('the_content', 'add_alt_tags', 99999);

$string = eregi_replace('([_\.0-9a-z-]+@([0-9a-z][0-9a-z-]+\.)+[a-z]{2,3})','<a href="mailto:\\1">\\1&lt;/ a>', $text); 
echo $string;

This function uses the power of regexp to check if some bad word are on the text, also offers the posibility to change those word for something else.

function filtrado($texto, $reemplazo = false) 
{ 
    $filtradas = 'ding, shit'; //Define here your words to censor separated by comma 
    $f = explode(',', $filtradas); 
    $f = array_map('trim', $f); 
    $filtro = implode('|', $f); 
    return ($reemplazo) ? preg_replace("#$filtro#i", $reemplazo, $texto) : preg_match("#$filtro#i", $texto) ; 
}

A simple PHP regex of validating a telephone number using regular expressions

$string = "(232) 555-5555"; 
if (preg_match('/^\(?[0-9]{3}\)?|[0-9]{3}[-. ]? [0-9]{3}[-. ]?[0-9]{4}$/', $string)) { 
    echo "example 2 successful."; 
}

Replace all links within href part of anchor in a HTML-String.

$pattern = '/(?<=href\=")[^]]+?(?=")/'; 
$replacedHrefHtml = preg_replace($pattern, $replacement, $html);

This is a awesome PHP regex for full email validation.

$regex = "([a-z0-9_.-]+)". # name 
    "@". # at 
    "([a-z0-9.-]+){2,255}". # domain & possibly subdomains 
    ".". # period 
    "([a-z]+){2,10}"; # domain extension 
$eregi = eregi_replace($regex, '', $email); 
$valid_email = empty($eregi) ? true : false;

$string = "255.255.255.255"; 
if (preg_match( 
    '/^(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]\d|\d)(?:[.](?:25[0-5]|2[0-4]\d|1\d\d|[1-9]\d|\d)){3}$/', 
    $string)) { 
    echo "IP address is good."; 
}

$string = "12345-1234"; 
if (preg_match('/^[0-9]{5}([- ]?[0-9]{4})?$/', $string)) { 
    echo "zip code checks out"; 
}

$text = "Sample sentence from KomunitasWeb, regex has become popular in web programming. Now we learn regex. According to wikipedia, Regular expressions (abbreviated as regex or regexp, with plural forms regexes, regexps, or regexen) are written in a formal language that can be interpreted by a regular expression processor"; 
$text = preg_replace("/\b(regex)\b/i", '<span style="background:#5fc9f6"&;gt;\1</ span>', $text); 
echo $text;

$url = "http://example.com/index.html"; 
preg_match('@^(?:http://)?([^/]+)@i', $url, $matches); 
$host = $matches[1]; 
echo $host;

$url = "http://example.com/"; 
if (preg_match('/^(http|https|ftp):\/\/([A-Z0-9][A-Z0-9_-]*(?:\.[A-Z0-9][A-Z0-9_-]*)+):?(\d+)?\/?/i', $url)) { 
    echo "Your url is ok."; 
} else { 
    echo "Wrong url."; 
}

function create_slug($string){ 
    $slug=preg_replace('/[^A-Za-z0-9-]+/', '-', $string); 
    return $slug; 
} 
echo create_slug('does this thing work or not');  

if (!preg_match("/^(http|https|ftp):/", $_POST['url'])) { 
    $_POST['url'] = 'http://'.$_POST['url']; 
}