s/\b\w+\s+nice\b/A nice/g;

[download]
or 
$w = 'nice'; $r = 'A';
s/\b\w+(\s+$w)\b/$r$1/g;

[download]
should work

In reply to Re: How to substitute all the words before a certain word in the middle of a line? by vkonovalov
in thread How to substitute all the words before a certain word in the middle of a line? by Anonymous Monk

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