How do I deny some keywords on user input - php

Question

Example user input that should be denied:
House for sale
Car for rent
WTB iphone with cheap price
How do I make my code deny inputs like those above?
$title = array('rent','buy','sale','sell','wanted','wtb','wts');
$user_title = stripslashes($_POST['title']);
if (in_array($user_title, $title)) {
$error = '<p class="error">Do not include ' . $user_title . ' on your title</p>';
}

Answer 1

If you want your denied words to be complete words and not just part of another word for it to be considered denied, you can use a regex based solution with word boundaries:
// array of denied words.
$deniedWords = array('rent','buy','sale','sell','wanted','wtb','wts');
// run preg_quote on each array element..as it may have a regex meta-char in it.
$deniedWords = array_map('preg_quote',$deniedWords);
// construct the pattern as /(\bbuy\b|\bsell\b...)/i
$pat = '/(\b'.implode('\b|\b',$deniedWords).'\b)/i';
// use preg-match_all to find all matches
if(preg_match_all($pat,$user_title,$matches)) {
// $matches[1] has all the found word(s), join them with comma and print.
$error = 'Do not include ' . implode(',',$matches[1]);
}
Ideone Link

Answer 2

You can use stripos():
$title = array('rent','buy','sale','sell','wanted','wtb','wts');
$user_title = stripslashes($_POST['title']);
foreach($title as $word)
{
if (stripos($user_title, $word) !== false)
{
$error = '<p class="error">Do not include ' . $word . ' on your title</p>';
break;
}
}

Answer 3

You can also use regex:
if (preg_match("/(rent|buy|sale|sell|wanted|wtb|wts)/is", $user_title)) {
$error = '<p class="error">Do not include ' . $user_title . ' on your title</p>';
}

Answer 4

You can utilize explode() in order to separate the words in $user_title and check each one to ensure it does not exist in $title.
$invalidWords = '';
$words = explode(' ', stripslashes($_POST['title']));
foreach($words as $word) {
if (in_array($word, $title)) {
$invalidWords .= ' ' . $word;
}
}
if (!empty($invalidWords)) {
echo '<p class="error">Do not include the following words in your title: ' . $invalidWords . '</p>';
}
RegEx is probably best, but off-hand I cannot easily figure out the expression required in order for you to be able to output all of the invalid words in a list to the user.