>I'm using the following code:
>
>str3=""
>
>If not empty(this.value)
>str3 = "upper(client.surname) like '" + allt(upper(this.value)) + "%'"
>Endif

This may be a workaround - just forbid square brackets and let them keep the apostrophes:

If not empty(this.value)
str3 = "upper(client.surname) like [" + allt(upper(this.value)) + "%]"
Endif

and in the textbox keypress, add this

if inlist (nKeyCode, 91, 93)
nodefault
endif

Nobody I know uses chr(91) or chr(93) as regular part of the name, and they are just regular string delimiters in VFP.