Java Regex Capturing Groups

Question

I am trying to understand this code block  In the first one  what is it we are looking for in the expression   My understanding is that it is any character  0 or more times    followed by any number between 0 and 9  one or more times    followed by any character  0 or more times      When this is executed the result is   Found value  This order was placed for QT3000  OK  Found value  This order was placed for QT300 Found value  0   Could someone please go through this with me   What is the advantage of using Capturing groups   import java util regex Matcher  import java util regex Pattern   public class RegexTut3        public static void main String args              String line    This order was placed for QT3000  OK             String pattern           d                     Create a Pattern object         Pattern r   Pattern compile pattern               Now create matcher object          Matcher m   r matcher line            if  m find                  System out println  Found value      m group 0                System out println  Found value      m group 1                System out println  Found value      m group 2              else               System out println  NO MATCH

User · Answer

From the doc    Capturing groups lt  a gt  are indexed from left    to right  starting at one   Group zero denotes the entire pattern  so    the expression m group 0  is equivalent to m group      So capture group 0 send the whole line

User · Answer

This is totally OK    The first group  m group 0   always captures the whole area that is covered by your regular expression  In this case  it s the whole string  Regular expressions are greedy by default  meaning that the first group captures as much as possible without violating the regex  The        d    the first part of your regex  covers the    QT300 int the first group and the 0 in the second  You can quickly fix this by making the first group non-greedy  change      to          For more info on greedy vs  lazy  check this site

User · Answer

Your understanding is correct  However  if we walk through         will swallow the whole string  it will need to give back characters so that    d   is satistifed  which is why 0 is captured  and not 3000   the last      will then capture the rest    I am not sure what the original intent of the author was  however

User · Answer

The issue you re having is with the type of quantifier  You re using a greedy quantifier in your first group  index 1 - index 0 represents the whole Pattern   which means it ll match as much as it can  and since it s any character  it ll match as many characters as there are in order to fulfill the condition for the next groups     In short  your 1st group    matches anything as long as the next group   d  can match something  in this case  the last digit     As per the 3rd group  it will match anything after the last digit    If you change it to a reluctant quantifier in your 1st group  you ll get the result I suppose you are expecting  that is  the 3000 part    Note the question mark in the 1st group     String line    This order was placed for QT3000  OK    Pattern pattern   Pattern compile          d          Matcher matcher   pattern matcher line   while  matcher find          System out println  group 1      matcher group 1        System out println  group 2      matcher group 2        System out println  group 3      matcher group 3        Output    group 1  This order was placed for QT group 2  3000 group 3    OK    More info on Java Pattern here    Finally  the capturing groups are delimited by round brackets  and provide a very useful way to use back-references  amongst other things   once your Pattern is matched to the input    In Java 6 groups can only be referenced by their order  beware of nested groups and the subtlety of ordering     In Java 7 it s much easier  as you can use named groups

[java] Java Regex Capturing Groups

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