解释Java正则表达式构造“ re？”。
子表达式/元字符“re？”匹配前面的表达式的0或1。
例子1
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class RegexExample {
public static void main( String args[] ) {
String regex = "Wel?";
String input = "Welcome to Nhooo";
Pattern p = Pattern.compile(regex);
Matcher m = p.matcher(input);
int count = 0;
while(m.find()) {
count++;
}
System.out.println("Number of matches: "+count);
}
}输出结果
Number of matches: 1
例子2
以下Java程序从用户接受一个字符串，验证它是否包含字母（两种情况），还接受数字。
import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class Test {
public static void main( String args[] ) {
String regex = "[a-zA-Z][0-9]?";
Scanner sc = new Scanner(System.in);
System.out.println("Enter an input string: ");
String input = sc.nextLine();
//Creating a Pattern object
Pattern p = Pattern.compile(regex);
//Creating a Matcher object
Matcher m = p.matcher(input);
if(m.find()) {
System.out.println("Match occurred");
} else {
System.out.println("Match not occurred");
}
}
}输出1
Enter an input string: sample text Match occurred
输出2
Enter an input string: sample text 34 56 Match occurred
输出3
Enter an input string: 32 89 45 63 Match not occurred