正则表达式替换非转义行情

Question

After having some trouble building a json string I discovered some text in my database containing double quotes. I need to replace the quotes with their escaped equivalents. This works:

function escape( $str ) {
    return preg_replace('/"/',"\\\"",$str);
}

but it doesn't take into account that a quote may already be escaped. How can I modify the expression so that it's only true only for a non escaped character?

Answer 1

You need to use a negative lookbehind here

function escape( $str ) {
    return preg_replace('/(?<!\\)"/',"\\\"",$str);
}