PHP - Preg_replace Issues! Help!

I have no idea what the problem is with preg_replace and strings smaller than 3 characters. I'm writing to do a pattern search of abreviations(array) and replacements (array)... tried word boundries, which seems not to work at all. (it is VITAL that im able to match the whole word)
here is a quick example which I cannot understand maybe someone could explain to me:
1. why repitions of certain abrevs do not get replaced.
2. why '/\bs.\b/' wont get matched (where there is a space before  the b. - hense the word boundry)

here is a snippet of code could someone please explain why this happens:

Code: [Select]
<?php
  $string='de d. s. pres. div. de de d. s.';
 
//DEFINE PATTERN
$patterns = array();
$patterns[0] = '/ s. /';
$patterns[1] = '/ b. /';
$patterns[2] = '/ pres./';
$patterns[3] = '/ Ind./';
$patterns[4] = '/ Mar./';
$patterns[5] = '/ Agt./';  
$patterns[6] = '/ Agy./';  
$patterns[7] = '/ Indpls./';                            
$patterns[8] = '/ Chgo./';
$patterns[9] = '/ Corp./';
$patterns[10] = '/ U./';    
$patterns[11] = '/ Fin./';          
$patterns[12] = '/ ls./';   
$patterns[13] = '/ Pres. /';    
$patterns[14] = '/ Internat. /';   
$patterns[15] = '/ m. /';
$patterns[16] = '/ N.Y.C./';   
$patterns[17] = '/ div. /';   
$patterns[18] = '/ de /';  
$patterns[19] = '/ d. /';    


//DEFINE PATTERN REPLACE
$replacements = array();
$replacements[0] = ' son of ';
$replacements[1] = ' born in ';
$replacements[2] = ' president';
$replacements[3] = ' Indiana ';
$replacements[4] = ' March ';
$replacements[5] = ' Agent ';  
$replacements[6] = ' Agency ';  
$replacements[7] = ' Indianapolis ';                            
$replacements[8] = ' Chicago';
$replacements[9] = ' Corporation ';
$replacements[10] = ' U '; 
$replacements[11] = ' Financial ';       
$replacements[12] = ' Island';   
$replacements[13] = ' President '; 
$replacements[14] = ' International ';  
$replacements[15] = ' married to ';
$replacements[16] = ' New York City ';   
$replacements[17] = ' divorced';    
$replacements[18] = ' de '; 
$replacements[19] = ' daughter of ';    

 echo '------ Original ---------<br>'.$string.'<br>';
$data = preg_replace($patterns, $replacements, $string);
 echo '<br>------ Abbreviation Replace ---------<br>'.$data.'<br>';  
?>

OUTPUT: ------ Original ---------
de d. s. pres. div. de de d. s.

------ Abbreviation Replace ---------
de daughter of son of president divorcedde daughter of d. s.

any help is appriciated thanks.