Hi, Please see the link below:
http://support.sas.com/documentation/cdl/en/lrdict/64316/HTML/default/viewer.htm#a000212246.htm
k means Keep
f means underscore character and English letters
s means space characters (blank, horizontal tab, vertical tab, carriage return, line feed, and form feed) to the list of characters

so, kfs means only keeps underscore character and English letters and space characters.

Hope it helps,
Yang

什么意思？也是compress这个函数实现吗？compress（var，pexchange ）这样吗？

var=prxchange("s/([\w\s]+)(.+)/\1/",-1,var);