CODEWITHSUNDEEP
javajavacc
peach
peach

Reputation: 13

How do you remove double quotes from the image of a JavaCC token?

In JavaCC, I'm accepting strings that are under the condition:

   < STRING : "\"" ("\\" ~[] | ~["\"","\\"] )* "\"" >

So the image ends up printing anything that is a string but with another set of double quotes.

For example, I'll input: "This is a sentence."

And the value will result : ""This is a sentence."" stored in a String variable.

Is there a way in Java to remove the extra set of double quotes so that it only prints: "This is a sentence."?

Upvotes: 1

Views: 567

Answers (2)

Theodore Norvell
Theodore Norvell

Reputation: 16231

If the input matched by your token is "Hello" then the value of the image field of the token will be a 7 character string whose first and last characters are double quote characters. They're not really extra they were they in the input. Say you write

void foo() : {
    Token t ; }
{
    t = <STRING>
    { System.out.println( t.image ) ; }
}

That'll print 7 characters and then a newline.

Now if you don't want those characters, well, @Bryan's answer will do it.

void foo() : {
    Token t ; }
{
    t = <STRING>
    { { String nakedImage = t.image.substring(1,t.image.length()-1) ;
        System.out.println( nakedImage ) ; } }
}

It should be noted that no quotes are removed. String objects in Java are immutable, meaning they can't be changed. What really happens is that a new String object gets created and a reference to it is assign to the nakedImage variable. The String object that t.image is a reference to remains the same.

Now you still have the problem of dealing with the back slashes. If the input is "Hello\tWorld", then t.image will be 14 characters long and nakedImage will be 12 characters long. What I do at this point is to run the string through a function builds a new string that has single characters where the nakedImage has escape sequences. So the result of that function on this example would be 11 characters long.

void foo() : {
    Token t ; }
{
    t = <STRING>
    { { String nakedImage = t.image.substring(1,t.image.length()-1) ;
        String unescapedImage = unescape( nakedImage ) ;
        System.out.println( unescapedImage ) ; } }
}

Here's such a function, based on one I wrote for a Java compiler.

private static String unescape( String str ) {
    StringBuffer result = new StringBuffer() ;
    for( int i=0, len = str.length() ; i<len ; ) {
        char ch = str.charAt(i) ;

        // Set ch and increment i ;
        if( ch == '\\' )  {
            ch = str.charAt(i+1) ;
            switch( ch ) {
                case 'b' : ch = '\b' ; i += 2 ; break ;
                case 't' : ch = '\t' ; i += 2 ; break ;
                case 'n' : ch = '\n' ; i += 2 ; break ;
                case 'f' : ch = '\f' ; i += 2 ; break ;
                case 'r' : ch = '\r' ; i += 2 ; break ;
                case '"' : case '\'' : case '\\' : i+= 2 ; break ;
                default: 
                    /*TODO Deal with errors. */ } }
        else {
            i += 1 ; }
        result.append( ch ) ; }
    return result.toString() ;
}

Upvotes: 2

Bryan
Bryan

Reputation: 169

str = str.substring(1,str.length()-1)

alternate for Javacc

Parsing Strings with JavaCC

Upvotes: 0

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