Substr and HTML tags - Problem...

Hi, I got a small problem: I need to limit a teaser-text via substr($teaser,0,100). The Problem I now have is, if the last 4 string chars are a html tag like <br> and this tag will be cut by the substr to something like that: <br - it will mess up the webpage... How can I avoid that the html tags are cut by the substr function? Is there a way to make the function know, that it has to leave html tags uncut?? Thanks in advance... Sascha Ragtschaa