I would search for a number followed by a char followed by another number using a regexp, and then filter in perl:

#!/usr/bin/perl

use strict;
use warnings;
use feature 'say';

my $text='abc 1foo 1x1 ee2 3B4 n 5 6t7 q9q 5+6';
while ($text=~/(\d)([[:alpha:]])(\d)/g) {
        if ($1 + 1 == $3) {
                say "$1$2$3 is wanted because $1 + 1 = $3";
        } else {
                say "$1$2$3 matches, but is not wanted ($1 + 1 != $3)"
+;
        }
}
[download]

Output:

1x1 matches, but is not wanted (1 + 1 != 1)
3B4 is wanted because 3 + 1 = 4
6t7 is wanted because 6 + 1 = 7
[download]

Alexander