sql server - PHP - Replace Tabbed whitespace with preg replace? -

i have form accepts tabbled white space " " example. want allow 1 space between words.

currently looks this:

preg_replace ("#[^0-9a-z]#i", " ",$string); 

remember need allow 1 space @ time, how fix this?

replace multiple whitespaces single space

$string = preg_replace('/\s\s+/',' ', $string);