Thank You for your help and patience and replying again. I have to excuse for my formal mistakes in the update, I should have read "How do I change ..." before. I have changed / replaced my script with your solution and I have created a small testfile

This is my script now: 

my $t = do { local $/;  <>; }; 

my $pre  = qr{ \S+ \s }xms;
 my $post = qr{ \s [^\n]+ \n [^\{]+ \{ }xms;
 ;;
 $t =~ s{ ^ $pre b $post \K Weiss \b}
                {Schwarz}xmsg;
 
 print qq{$t};

[download]

This was the input:

..........
[FEN "1B3k2/2R1b3/4p1pp/2r1P1p1/6P1/7P/5PK1/8 w - - 4 34"]
{Weiss am Zug}  *

..........
[FEN "1B6/1Q4bk/R5p1/1pp4p/8/1n5P/1P4PK/1q6 b - - 2 42"]
{Weiss am Zug}  *

...........
[FEN "1Bn4r/1k3q2/1pp1bbp1/1N1p1p1r/Q2Pp3/6PN/PP2PPB1/K1R4R b - - 0 23
+"]
{Weiss am Zug}  *

...........
[FEN "1K6/4kp2/1P4p1/7p/7P/1r3PP1/1p6/1R6 w - - 3 52"]
{Weiss am Zug}  *

[download]

The output was exactly the same, Weiss was not changed.

In reply to Re^2: Regex with condition by OldChamp
in thread Regex with condition by OldChamp

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.