Yet another approach --general, but neither brief nor elegant in exchange for providing (IMO) a clear set of steps with some explanation of the key regexen. 
#!/usr/bin/perl
use 5.016;
use strict;
use warnings;

# 1040996

my @strs = ("foo bar >lt;amp; blivitz", 
            "FOO BAR BLIVITZ, >apos; "sect; ",
            "no entities here",
            "But there are ¢pound; entities for 'cent' and 'pound
+' here.",
            );

for my $str(@strs) {
    if ( $str =~ /(&[^ ]+)/ ) {   # match any ampersand followed by on
+e or 
                                  # more NON-spaces (aka \S; see below
+)
        my $found = $1;
        say "DEBUG: found semicolon(s) at |$found| in \"$str\"";
        if ($str =~ /
                [^&;]*?     # anything that's neither "&" nor ";"
                (&.+)       # followed by an ampersand and multiple ch
+ars
                (?!\S)      # until prev capture is followed by someth
+ing 
                            # NOT-a-space ("negative lookahead")
                /gx )    {  # globally, extended notation, end conditi
+ons, begin actions
            my $substr = $1;
            say "\$substr: $substr\n";
            (my $fixed = $substr ) =~ s/(;)([a-z])/$1&$2/g;
            say "\$fixed: $fixed \n";
        }
    } else {
        say "\n\t No html entities found.\n";
    }
}

=head

C:\>1040996.pl
DEBUG: found semicolon(s) at |>lt;amp;| in "foo bar >lt;
+amp; blivitz"
$substr: >lt;amp; blivitz

$fixed: &><& blivitz

DEBUG: found semicolon(s) at |>apos;| in "FOO BAR BLIVITZ, >apos
+; "sect; "
$substr: >apos; "sect;

$fixed: >' "§


         No html entities found.

DEBUG: found semicolon(s) at |¢pound;| in "But there are ¢po
+und; entities for 'cent' and 'pound' here."
$substr: ¢pound; entities for 'cent' and 'pound' here.

$fixed: ¢£ entities for 'cent' and 'pound' here.


=cut

[download]

BUT... this is imperfect (note the captured, trailing "blivitz" in the first example, and the loss of non-entity material at the beginning of each array element. That's easily enough fixed with a little more code.

ANOTHER BUT: This breaks on some edge cases... such as a line where the bad entities immediately precede the EOL.

Update: added word "general" in first graf.


If you didn't program your executable by toggling in binary, it wasn't really programming!

In reply to Re: Question for regex experts by ww
in thread Question for regex experts by evgen-i

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