java util regex - importance of Pattern compile

Question

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

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Pre-compiling the regex increases the speed  Re-using the Matcher gives you another slight speedup  If the method gets called frequently say gets called within a loop  the overall performace will certainly go up

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It is matter of performance and memory usage  compile and keep the complied pattern if you need to use it a lot  A typical usage of regex is to validated user input  format   and also format output data for users  in these classes  saving the complied pattern  seems quite logical as they usually called a lot   Below is a sample validator  which is really called a lot      public class AmountValidator         Accept 123 - 123 456 - 123 345 34     private static final String AMOUNT REGEX    d 1 3     d 3         d 1 4         d 1 4          Compile and save the pattern       private static final Pattern AMOUNT PATTERN   Pattern compile AMOUNT REGEX         public boolean validate String amount             if   AMOUNT PATTERN matcher amount  matches                  return false                         return true                As mentioned by  Alan Moore  if you have reusable regex in your code   before a loop for example   you must compile and save pattern for reuse

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When you compile the Pattern Java does some computation to make finding matches in Strings faster   Builds an in-memory representation of the regex   If you are going to reuse the Pattern multiple times you would see a vast performance increase over creating a new Pattern every time   In the case of only using the Pattern once  the compiling step just seems like an extra line of code  but  in fact  it can be very helpful in the general case

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Compile parses the regular expression and builds an in-memory representation  The overhead to compile is significant compared to a match  If you re using a pattern repeatedly it will gain some performance to cache the compiled pattern

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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

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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  lt  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  lt  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

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The compile   method is always called at some point  it s the only way to create a Pattern object   So the question is really  why should you call it explicitly   One reason is that you need a reference to the Matcher object so you can use its methods  like group int  to retrieve the contents of capturing groups   The only way to get ahold of the Matcher object is through the Pattern object s matcher   method  and the only way to get ahold of the Pattern object is through the compile   method   Then there s the find   method which  unlike matches    is not duplicated in the String or Pattern classes    The other reason is to avoid creating the same Pattern object over and over   Every time you use one of the regex-powered methods in String  or the static matches   method in Pattern   it creates a new Pattern and a new Matcher   So this code snippet   for  String s   myStringList        if   s matches    d                doSomething                 is exactly equivalent to this   for  String s   myStringList        if   Pattern compile    d    matcher s  matches               doSomething              Obviously  that s doing a lot of unnecessary work   In fact  it can easily take longer to compile the regex and instantiate the Pattern object  than it does to perform an actual match   So it usually makes sense to pull that step out of the loop   You can create the Matcher ahead of time as well  though they re not nearly so expensive   Pattern p   Pattern compile    d     Matcher m   p matcher      for  String s   myStringList        if   m reset s  matches               doSomething              If you re familiar with  NET regexes  you may be wondering if Java s compile   method is related to  NET s RegexOptions Compiled modifier  the answer is no   Java s Pattern compile   method is merely equivalent to  NET s Regex constructor   When you specify the Compiled option   Regex r   new Regex    d    RegexOptions Compiled         it compiles the regex directly to CIL byte code  allowing it to perform much faster  but at a significant cost in up-front processing and memory use--think of it as steroids for regexes   Java has no equivalent  there s no difference between a Pattern that s created behind the scenes by String matches String  and one you create explicitly with Pattern compile String     EDIT  I originally said that all  NET Regex objects are cached  which is incorrect   Since  NET 2 0  automatic caching occurs only with static methods like Regex Matches    not when you call a Regex constructor directly  ref

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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

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

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