[Analogpoint]

The dots in your censor word b.s. are being interpreted as the "any single character" expression, which is a dot (.) in the regular expression. What you need to do is escape any characters that have a special meaning for regexes, in your $censorword, before calling preg_replace In PHP, you can use preg_quote to do this.

$text = preg_replace('#(?<=[^a-z]|^)(' . preg_quote($censorword, '#') . ')(?=[^a-z]|$)#si', "[$censorword]", $text);