Have a look at perlre which helps me all the time!
Especially the part Extended Patterns (which I've used here) is very interesting!

For the string you submitted the following will do
$str =~ s/(?<=\s)TXT/FOO/g ;

[download]
CHeers
LuCa

UPDATE: this one might be easier 
$str =~ s/ TXT/ FOO/g ;

[download]

In reply to Re: Construct a REGEX by selected substitution by jeanluca
in thread Construct a REGEX by selected substitution by pysome

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