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Best way/algorithm to find out if a string consists of only a given set of characters
Input 8888338385
     Output VALID
Input 887837348234
Output : Invalid

public boolean isValidString(String str) {
  return str.matches("[358]*");
}

import java.util.regex.Pattern;

public class Matcher {
  private Pattern pattern;

  public Matcher() {
    this.pattern = Pattern.compile("[358]*");
  }

  public isValid(String str) {
    return pattern.matcher(str).matches();
  }
}

const char* haystack = "8888338385";
const char* filter = "385";

if (strlen(haystack) != strspn(haystack, filter))
{
  // oops - haystack contains more characters...
}