Java中的pattern和^ pattern $ regEx有什么区别？

Question

What is the difference between following two regEx ? both matches exact string in java.

System.out.println("true".matches("true"));
System.out.println("true".matches("^true$")); // ^ means should start and $ means should end. So it should mean exact match true. right ?

Both prints true .

Answer 1

You won't be able to see the difference in your selected string.

Try using: - "afdbtrue" or "tru" with both of them. Both strings would not match the first pattern.

^true* -> This means the string should start with t ( Caret(^) means start of string), followed by r and u , and there can be 0 or more e after tru ( u* means 0 or more u)

System.out.println("tru".matches("^true*"));     // true
System.out.println("trueeeee".matches("^true*"));// true
System.out.println("asdtrue".matches("^true*")); // false

System.out.println("tru".matches("true"));       // false
System.out.println("truee".matches("true"));   // false
System.out.println("asdftrue".matches("true"));  // false

• Your first and second sysout will print true , because tru starts with t and there is 0 e after tru . Same with trueee . That will be fine
• Your 3rd sysout will print false , because asdtrue does not start with t
• Your 4th sysout, will again pring false because it is not exactly true
• Your 5th and 6th sysouts will again print false , because they does not exactly matches true

---

UPDATE : -

After OP changed the question: -

• ^(caret) matches at the start of the string
• $(Dollar) matches at the end of the string.

So, ^true$ will match the string with starting with true and ending with true . So, now in this case, there won't be any difference between, true and ^true$ in the way you are using.

str.matches("true") will match a string that is exactly "true" ., and str.matches("^true$") will also match exactly "true" , because it starts with and ends with "true" .

System.out.println("true".matches("^true$"));     // true
System.out.println("This will not match true".matches("^true$"));   // false
System.out.println("true".matches("true"));       // true
System.out.println("This will also not match true".matches("true")); // false

---

UPDATE : -

However, if you use Matcher.find method, then there will be a difference in the two pattern. Try this: -

    Matcher matcher = Pattern.compile("true").matcher("This will be true");
    Matcher matcher1 = Pattern.compile("^true$").matcher("This won't be true"); 

    if (matcher.find()) {  // Will find
        System.out.println(true);
    } else {
        System.out.println(false);
    }
    if (matcher1.find()) {  // Will not find
        System.out.println(true);
    } else {
        System.out.println(false);
    }

OUTPUT : -

true
false

• See Matcher documentation for more details on these methods. ( Matcher#find and Matcher#matches )

Answer 2

^ represents the start of the string

$ represents end of the string

matches method Returns only true if the WHOLE string can be matched

So,both "true" & "^true$" would match only a single word ie true

---

But if you use find method then following would be valid So, "true" would match those lines that contain true anywhere

Is it true//match
How true//match
true is truth//match
not false//no match

"^true$" would match any line that that has only one word that is true

true//match
true is truth//no match
it is true//no match