RegEx trouble

I copy&paste texts into ArticleText field (rich text) from word documents. Due to text is always long, I create for a ArticleText View - ArticlePreview formula field, which should keep first 100 symbols of ArticleText field.

Formula calls function getPreviewText(ArticleText, 100)

Due to value of ArticleText is not a clean text, but html formated text, so I need to clean it from tags by

string getPreviewText(string srcText, int len)
{
str = "<(.|\n)*?>";
input.srcText = (input.srcText).replaceAll(str,"");

it works perfectly excluding

<!--[if gte mso 9]><xml> <w:WordDocument>  <w:View>Normal</w:View>  <w:Zoom>0</w:Zoom>
<w:LidThemeAsian>X-NONE</w:LidThemeAsian>
 </xml><![endif]-->

which brings me Normal, 0, X-NONE, etc garbage.
I try to get rid of it by code:
str = "()";
input.srcText = (input.srcText).replaceAll(str,"");

which I put before tag cleaning code.
But as soon as I add this code into function, my form is freezing after pushing Submit data (Please wait ...)

I think my regex is doing something wrong. Are there any ideas?

Thanks in advance

Andrey