Similar to tobyink's solution but doing a global match in case there are multiple "fred"s on your intervals. It seems to work except that it matches twice at each point, probably for similar reasons as explored here.

]$ perl -Mstrict -Mwarnings -E '
$_ = q{abcdefredfghfredijklmnopfredqrs};
for my $n ( 4, 5 )
{
    say qq{\$n = $n};
    say qq{Matched $1 at position @{ [ pos( $_ ) ] }} while
       m{\G(?:.{$n})*?(?=(fred.*))}g;
}'
$n = 4
Matched fredijklmnopfredqrs at position 12
Matched fredijklmnopfredqrs at position 12
Matched fredqrs at position 24
Matched fredqrs at position 24
$n = 5
Matched fredfghfredijklmnopfredqrs at position 5
Matched fredfghfredijklmnopfredqrs at position 5
$

[download]

I hope this is useful.

Cheers,

JohnGG

In reply to Re: A regex that only matches at offset that are multiples of a given N? by johngg
in thread A regex that only matches at offset that are multiples of a given N? by BrowserUk

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