Regular Expressions : Find mismatched point (or char index)

Question

I'm beginner to Regular Expressions. Is there any way to find mismatched point or char index when we validating string using Regular Expressions? I've used RegEx in Java to validate string.
I only need to find first mismatched index.
Update
Please consider example like this.
Regular Expression : ^\d{9}[VX]$
Accepted String : 547812375X
Wrong String : 547A12375X

In wrong string there's A instead of 8. What I need is find the mismatched index, for here is 4. Character at index is mismatched for RegEx.

Answer 1

I think this code might do what you want:

package so7783938;

import static org.junit.Assert.assertEquals;

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

import org.junit.Test;

public class RegexFailureTest {

  public static int firstFailurePoint(Pattern regex, String str) {
    for (int i = 0; i <= str.length(); i++) {
      Matcher m = regex.matcher(str.substring(0, i));
      if (!m.matches() && !m.hitEnd()) {
        return i - 1;
      }
    }
    if (regex.matcher(str).matches()) {
      return -1;
    } else {
      return str.length();
    }
  }

  @Test
  public void testSimple() {
    Pattern abc = Pattern.compile("abc");
    assertEquals(0, firstFailurePoint(abc, ""));
    assertEquals(1, firstFailurePoint(abc, "a"));
    assertEquals(2, firstFailurePoint(abc, "ab"));
    assertEquals(-1, firstFailurePoint(abc, "abc"));
    assertEquals(3, firstFailurePoint(abc, "abcd"));
    assertEquals(3, firstFailurePoint(abc, "abcdefghi"));
    assertEquals(1, firstFailurePoint(abc, "aaa"));
    assertEquals(2, firstFailurePoint(abc, "abb"));
  }

  @Test
  public void testAlternative() {
    Pattern regex = Pattern.compile("hello|world");
    assertEquals(0, firstFailurePoint(regex, "x"));
    assertEquals(-1, firstFailurePoint(regex, "hello"));
    assertEquals(-1, firstFailurePoint(regex, "world"));
    assertEquals(3, firstFailurePoint(regex, "hel"));
    assertEquals(5, firstFailurePoint(regex, "hello kitty"));
    assertEquals(3, firstFailurePoint(regex, "help me"));
    assertEquals(3, firstFailurePoint(regex, "worse is better"));
  }

  @Test
  public void testExample() {
    Pattern regex = Pattern.compile("^\\d{9}[VX]$");
    assertEquals(-1, firstFailurePoint(regex, "547812375X"));
    assertEquals(3, firstFailurePoint(regex, "547A12375X"));
  }

}