pattern.matcher() vs pattern.matches()

Question

I am wondering why the results of the java regex pattern.matcher() and pattern.matches() differ when provided the same regular expression and same string

String str = "hello+";
Pattern pattern = Pattern.compile("\\+");
Matcher matcher = pattern.matcher(str);

while (matcher.find()) {
    System.out.println("I found the text " + matcher.group() + " starting at " 
        + "index " + matcher.start() + " and ending at index " + matcher.end());
}

System.out.println(java.util.regex.Pattern.matches("\\+", str));

The result of the above are:

I found the text + starting at index 5 and ending at index 6
false

I found that using an expression to match the full string works fine in case of matches(".*\\+").

Answer 1

pattern.matcher(String s) returns a Matcher that can find patterns in the String s. pattern.matches(String str) tests, if the entire String (str) matches the pattern.

In brief (just to remember the difference):

• pattern.matcher - test if the string contains-a pattern
• pattern.matches - test if the string is-a pattern

Answer 2

If you add this println():

Matcher matcher = pattern.matcher(str);
System.out.println(matcher.matches());

It still prints false. The .matches() method (both Pattern.matches() and Matcher.matches()) will attempt to match the entire string; it's as if the regex had the ^ and $ anchors, while find() scans it for partial matches.

Answer 3

Matcher.find() attempts to find the next subsequence of the input sequence that matches the pattern.

Pattern.matches(String regex, CharSequence input) compiles the regex into a Matcher and returns Matcher.matches().

Matcher.matches attempts to match the entire region (string) against the pattern (Regex).

So, in your case, the Pattern.matches("\\+", str) returns a false since str.equals("+") is false.

Answer 4

Pattern.matches is testing the whole String, in your case you should use:

 System.out.println(java.util.regex.Pattern.matches(".*\\+", str));

Meaning any string and a + symbol

Answer 5

From the Javadoc, see the if, and only if, the entire region section

   /**
     * Attempts to match the entire region against the pattern.
     *
     * <p> If the match succeeds then more information can be obtained via the
     * <tt>start</tt>, <tt>end</tt>, and <tt>group</tt> methods.  </p>
     *
     * @return  <tt>true</tt> if, and only if, <b>the entire region</b> sequence
     *          matches this matcher's pattern
     */
    public boolean matches() {
        return match(from, ENDANCHOR);
    }

So if your String was just "+", you'd get a true result.

Answer 6

Well thanks all, I got it. Thanks for explaining the working of matcher.find() and matcher.matches(), now these english words find and matches makes more sense to me :).

Answer 7

I think your question should really be "When should I use the Pattern.matches() method?", and the answer is "Never." Were you expecting it to return an array of the matched substrings, like .NET's Matches methods do? That's a perfectly reasonable expectation, but no, Java has nothing like that.

If you just want to do a quick-and-dirty match, adorn the regex with .* at either end, and use the string's own matches() method:

System.out.println(str.matches(".*\\+.*"));

If you want to extract multiple matches, or access information about a match afterward, create a Matcher instance and use its methods, like you did in your question. Pattern.matches() is nothing but a wasted opportunity.