Here's a happy little regex: 
my $sentence = "This is a nice weather. He is a nice guy. 
We have some nice food in our dinner.";

$sentence =~ s:(\w+\s)+(\bnice\b.*?\.):A $2:g;

print "$sentence\n";

[download]
That looks for a word followed by a space, repeated any number of times, followed by the word nice (with word boundaries around it) followed by a minimal number of characters before a final period. It replaces the first bit with 'A' and keeps the second around. /g, of course, makes it look for all the matches in the string.

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

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.