Home / Questions / 69597
Accepted
deluf
Asked: 2020-05-13 12:45:33 +0800 CST 2020-05-13 12:45:33 +0800 CST

Match a letter at the beginning of the string

  • 772

I made the following code:

import java.util.regex.Matcher; 
import java.util.regex.Pattern; 
import java.lang.Math; // headers MUST be above the first class 

// one class needs to have a main() method 
public class HelloWorld 
{ 
    // arguments are passed using the text field below this editor 
    public static void main(String[] args) 
    { 

          String ori = "kdya"; 
          String pa = "[kd]"; 
          Pattern pat = Pattern.compile(pa); 
          Matcher mat = pat.matcher(ori); 
          System.out.println(mat.matches()); 

    } 
}

I want to match the start of the original String (the first character) with any character in the pattern list.

It doesn't match, but it does exist. It's wrong?

java

2 Answers

  1. The method Matcher#matches()tries to match a regex to a text completely, from beginning to end. There are practically no cases where one should use this method.

    Instead, for standard regular expression behavior, and to be able to match anywhere in the text, the Matcher#find().

    In regex, the special character ^matches the position at the beginning of the text (see in the description of Pattern ).

    import java.util.regex.Matcher; 
import java.util.regex.Pattern;

    String texto = "kdya"; 
String regex = "^[kd]"; 
Pattern pat  = Pattern.compile(regex); 
Matcher mat  = pat.matcher(texto);

if (mat.find())  // ⬅️⬅️⬅️ Se compara con .find()
{
    System.out.println("El texto '" + texto + "' empieza con " + mat.group()); 
}
else
{
    System.out.println("No coincide");
}

    Demo en ideone


    Note: I understand that this is a case you are practicing with. Otherwise, it's much simpler to use String#startsWith:

    if (texto.startsWith("k") || texto.startsWith("d")) {
    //...
}
    • 3
  2. Best Answer

    Your regex accepts strings of length 1 that contain k or s.
    Try with String original = "k";and you will see that it prints true.

    In order for your regex to accept any string that starts with k or by s you must use:

    [ks].*

[ks] Acepta un solo caracter de entre los especificados.
.    Acepta cualquier carácter. Pero solo 1. (*)
*    Modifica el elemento anterior (el .) haciendo que acepte
    cualquier cantidad de 0 a infinito.

    (*) Whether or not it accepts line terminators is configurable, but this is off topic.

    • 2