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Re: Replacing a specfied instance of a pattern in a string

by ikegami (Pope)
on Jul 14, 2005 at 21:32 UTC ( #475054=note: print w/ replies, xml ) Need Help??


in reply to Replacing a specfied instance of a pattern in a string

my $str = 'Fsih my test variable is fsihd or is it gfsih or gfsih +d fsih'; # 1111 2222 3333 + 4444 my $substr = 'fsih'; my $replace = 'fish'; my $instance = 3; # 3rd print("$str\n"); my $pos = -length($substr); for (;;) { last if !$instance--; $pos = index($str, $substr, $pos + length($substr)); last if $pos < 0; } substr($str, $pos, length($substr), $replace) if $pos >= 0; print("$str\n");

Update: Fixed off-by-one error.


Comment on Re: Replacing a specfied instance of a pattern in a string
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Re^2: Replacing a specfied instance of a pattern in a string
by krisahoch (Deacon) on Jul 14, 2005 at 21:35 UTC
    Now, is there a way to do it with Regular Expression?
      my $str = 'Fsih my test variable is fsihd or is it gfsih or gfsih +d fsih'; # 1111 2222 3333 + 4444 my $substr = 'fsih'; my $regexp = qr/\Q$substr\E/; my $replace = 'fish'; my $instance = 3; # 3rd print("$str\n"); my $pre_count = $instance - 1; $str =~ s/((?:$regexp.*?){$pre_count})$regexp/$1$replace/; print("$str\n");

      You might have noticed both of these are case sensitive. Both can be made case-insensitive. The non-regexp version is most likely much faster. The regexp version can handle regexps instead of constant strings.

Re^2: Replacing a specfied instance of a pattern in a string
by Roy Johnson (Monsignor) on Jul 14, 2005 at 22:01 UTC
    A tad simpler looping, and case-insensitive:
    my $foo = 'Fsih my test variable is fsihd or is it gfsih or gfsihd f +sih'; my $from = 'fsih'; my $which = 3; my $to = 'FISH'; # makes it easy to see $foo =~ /\Q$from\E/ig for (1..$which); substr($foo, pos($foo) - length($from), length($from), $to) if pos($fo +o); print "Foo = $foo\n";

    Caution: Contents may have been coded under pressure.
      That reminds me of /c!
      my $str = 'Fsih my test variable is fsihd or is it gfsih or gfsih +d fsih'; # 1111 2222 3333 + 4444 my $substr = 'fsih'; my $regexp = qr/\Q$substr\E/; my $replace = 'fish'; my $instance = 3; # 3rd print("$str\n"); scalar $str =~ /\G.*?$regexp/gc while --$instance; $str =~ s/\G(.*?)$regexp/$1$replace/; print("$str\n");

      The /c is needed in case there are less than $instance instances of the search string.

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