Regular expression to match unescaped special characters only

Question

I'm trying to come up with a regular expression that can match only characters not preceded by a special escape sequence in a string.

For instance, in the string Is ? stranded//? , I want to be able to replace the ? which hasn't been escaped with another string, so I can have this result : **Is Dave stranded?**

But for the life of me I have not been able to figure out a way. I have only come up with regular expressions that eat all the replaceable characters.

How do you construct a regular expression that matches only characters not preceded by an escape sequence?

Answer 1

Use a negative lookbehind, it's what they were designed to do!

(?<!//)[?]

To break it down:

(
    ?<!    #The negative look behind.  It will check that the following slashes do not exist.
    //     #The slashes you are trying to avoid.
)
[\?]       #Your special charactor list.

Only if the // cannot be found, it will progress with the rest of the search.

I think in Java it will need to be escaped again as a string something like:

Pattern p = Pattern.compile("(?<!//)[\\?]");