regex.PatternSyntaxException meta character '*'

Question

Scanner input = new Scanner(System.in);
String girdi = input.nextLine();
for(int i=1; i<girdi.length(); i+=1){
    if(girdi.charAt(i) == '*'){
        int temp1= girdi.charAt(i-1)-'0';
        int temp2= girdi.charAt(i+1)-'0';
        int temp3 = temp1 * temp2;
        String a = girdi.substring(i-1,i);
        String b = "" + temp3;                
        String a2 = girdi.substring(i+1,i+2);
        String b2 = girdi.substring(i , i+1); 
        girdi = girdi.replaceFirst(a,b);
        girdi = girdi.replaceFirst(b2, "");   
        girdi = girdi.replaceFirst(a2, "");     
    }
}

I am just trying to make multiplication

when I type something like that (3*5) give me that error:

Exception in thread "main" java.util.regex.PatternSyntaxException: Dangling meta character '*' near index 0

and when I am using backslah to escape :

if(s.charAt(i) == '\\*'){

I see unclosed character literal and illegal start of expression errors

what should I do?

No, it's not this part of the code doing that. No regexes here. — Andy Turner
– Andy Turner, Commented Nov 18, 2016 at 21:56
You're first example should not throw an exception, please edit your question to include your code in full. — Jonny Henly
– Jonny Henly, Commented Nov 18, 2016 at 21:56
Please edit your question and include all lines of the exception’s stack trace. — VGR
– VGR, Commented Nov 18, 2016 at 22:00
@Agarta what do you mean "give an input"? If s is "4 * 5", you've still got no regexes in the code you've shown. — Andy Turner
– Andy Turner, Commented Nov 18, 2016 at 22:03
@Agarta you need to show a minimal reproducible example. The code you have shown doesn't produce the error you describe. — Andy Turner
– Andy Turner, Commented Nov 18, 2016 at 22:12

Andy Turner · Accepted Answer · 2016-11-21 08:21:36Z

Refer to the Javadoc of String.replaceFirst:

The first parameter is a regular expression (meaning characters like * have special meaning);
The second parameter is a regex replacement (meaning certain character sequences have special meaning, e.g. \ and $).

If you want to pass in strings from an untrusted source (like user input), you need to quote them appropriately:

girdi = girdi.replaceFirst(Pattern.quote(a), Matcher.quoteReplacement(b));

However, String.replaceFirst is not the right tool for this job. It won't work correctly if there is a match for the search string before the one you are really looking for.

The most straightforward modification to your code would be to use String.substring - you know exactly where the thing you want to replace is, so make use of that information:

girdi = girdi.substring(0, i-1) + temp3 + girdi.substring(i+2);
i

(This would replace the last 7 lines in the loop)

You can do this better again by using a StringBuilder, to avoid creating unnecessary strings in the loop.

However, this still wouldn't work in the case of multi-digit or negative numbers.

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