need help with highlighting keywords with preg_replace function

[bnownlater]

I have the following code to highlight keywords that are found in a string. The only problem is that I only want to highlight the keywords when they are at the begining of a word. (Example: $keyword[0] = "test"... "testing", "test ", "tester", but not "attest", "newtest".) currently the following code highlights any occurance of the keywords. Thanks.

$excerptstr= trim(stripslashes($row['ABSTRACT']));

	If (isset($mykeyword) && trim($mykeyword)=="Go"){

       for($i=0;$i<count($keyword2);$i++){

		 If ($i==0){
		 	$excerptstr2=preg_replace("'($keyword2[$i])'si",
"<span class='highlight2'>\\1</span>",$excerptstr);
	         }else{
			  	$excerptstr2=preg_replace("'($keyword2[$i])'si",
"<span class='highlight2'>\\1</span>",$excerptstr2);
	         }
	       }
	       echo $excerptstr2;

    }else{
        echo $excerptstr;
    }

[dantone]

This works, but could be slow if the excerpt is large or you have a lot of keywords.

I don't doubt that there may be an easier way to do this that I am overlooking. After all ... it is Friday 😃

If (isset($mykeyword) && trim($mykeyword)=="Go"){

for($i=0;$i<count($keyword2);$i++){

//assign your value to $parsestr variable
If ($i==0){
$parsestr = $excerptstr;	 
}else{
$parsestr = $excerptstr2;
}
//done assigning $parsestr variable

//create an array of all the words in your excerpt
$strarr = split(" ", $parsestr);

//set the value of sentence var to empty
$sentence = "";

//for each word, check to see if it starts with the keyword
//then add it to the sentence
for ($j=0; $j<count($strarr); $j++) {
 	 $sentence .= ereg_replace("^($keyword2[$i])(.+)","<font color=red>\\1</font>\\2",$strarr[$j]) . " ";
}

//trim the space off of the end of the sentence
//assign the resulting excerpt to your excerptstr2 variable
$excerptstr2 = trim($sentence);
}

//print the excerpt
echo $excerptstr2;

}else{
     echo $excerptstr;
}

Dave