java.util.regex - importance of Pattern.compile()?

user_207647 0
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What is the importance of Pattern.compile() method?
Why do I need to compile the regex string before getting the Matcher object?

For example :

String regex = "((\\S+)\\s*some\\s*";

Pattern pattern = Pattern.compile(regex); // why do I need to compile
Matcher matcher = pattern.matcher(text);
regex java
7 Answers

Pattern.compile() allow to reuse a regex multiple times (it is threadsafe). The performance benefit can be quite significant.

I did a quick benchmark: 

    @Test
    public void recompile() {
        var before = Instant.now();
        for (int i = 0; i < 1_000_000; i++) {
            Pattern.compile("ab").matcher("abcde").matches();
        }
        System.out.println("recompile " + Duration.between(before, Instant.now()));
    }

    @Test
    public void compileOnce() {
        var pattern = Pattern.compile("ab");
        var before = Instant.now();
        for (int i = 0; i < 1_000_000; i++) {
            pattern.matcher("abcde").matches();
        }
        System.out.println("compile once " + Duration.between(before, Instant.now()));
    }

compileOnce was between 3x and 4x faster. I guess it highly depends on the regex itself but for a regex that is often used, I go for a static Pattern pattern = Pattern.compile(...)

user_35108600

Similar to 'Pattern.compile' there is 'RECompiler.compile' [from com.sun.org.apache.regexp.internal] where:
1. compiled code for pattern [a-z] has 'az' in it
2. compiled code for pattern [0-9] has '09' in it
3. compiled code for pattern [abc] has 'aabbcc' in it.

Thus compiled code is a great way to generalize multiple cases. Thus instead of having different code handling situation 1,2 and 3 . The problem reduces to comparing with the ascii of present and next element in the compiled code, hence the pairs. Thus
a. anything with ascii between a and z is between a and z
b. anything with ascii between 'a and a is definitely 'a'

user_56575200

Pattern class is the entry point of the regex engine.You can use it through Pattern.matches() and Pattern.comiple(). #Difference between these two. matches()- for quickly check if a text (String) matches a given regular expression comiple()- create the reference of Pattern. So can use multiple times to match the regular expression against multiple texts.

For reference:

public static void main(String[] args) {
     //single time uses
     String text="The Moon is far away from the Earth";
     String pattern = ".*is.*";
     boolean matches=Pattern.matches(pattern,text);
     System.out.println("Matches::"+matches);

    //multiple time uses
     Pattern p= Pattern.compile("ab");
     Matcher  m=p.matcher("abaaaba");
     while(m.find()) {
         System.out.println(m.start()+ " ");
     }
}
user_45932800
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