Java regex capturing groups indexes

Question

I have the following line   typeName  ABC xxxxx      I need to fetch the word ABC    I wrote the following code snippet   Pattern pattern4 Pattern compile           matcher pattern4 matcher typeName    String nameStr     if matcher find          nameStr matcher group 1        So if I put group 0  I get ABC  but if I put group 1  it is ABC  so I want to know    What does this 0 and 1 mean  It will be better if anyone can explain me with good examples   The regex pattern contains a   in it  so why group 1  result omits that  Does group 1 detects all the words inside the parenthesis   So  if I put two more parenthesis such as    s   d        then  will be there two groups  group 1  will return the   d   part and group 2  return the      part    The code snippet was given in a purpose to clear my confusions  It is not the code I am dealing with  The code given above can be done with String split   in a much easier way

User · Answer

Parenthesis    are used to enable grouping of regex phrases   The group 1  contains the string that is between parenthesis      so    in this case  And group 0  contains whole matched string   If you would have more groups  read         it would be put into groups with next indexes  2  3 and so on

User · Answer

Capturing and grouping  Capturing group  pattern  creates a group that has capturing property    A related one that you might often see  and use  is    pattern   which creates a group without capturing property  hence named non-capturing group   A group is usually used when you need to repeat a sequence of patterns  e g      w     or to specify where alternation should take effect  e g    0 1 1 0       then 0 1 or 1 0  then    versus  0 1 1 0    0 1 or 1 0     A capturing group  apart from grouping  will also record the text matched by the pattern inside the capturing group  pattern   Using your example            matches ABC and   matches    and since    is inside capturing group       the text ABC is recorded for the capturing group 1   Group number  The whole pattern is defined to be group number 0   Any capturing group in the pattern start indexing from 1  The indices are defined by the order of the opening parentheses of the capturing groups  As an example  here are all 5 capturing groups in the below pattern    group    non-capturing-group  g   ro u p   nested inside  another group    assertion                                                                            1-----1                                    4------4       5-------5                                                3---------------3                                              2-----------------------------------------2   The group numbers are used in back-reference  n in pattern and  n in replacement string   In other regex flavors  PCRE  Perl   they can also be used in sub-routine calls   You can access the text matched by certain group with Matcher group int group   The group numbers can be identified with the rule stated above   In some regex flavors  PCRE  Perl   there is a branch reset feature which allows you to use the same number for capturing groups in different branches of alternation   Group name  From Java 7  you can define a named capturing group    lt name gt pattern   and you can access the content matched with Matcher group String name   The regex is longer  but the code is more meaningful  since it indicates what you are trying to match or extract with the regex   The group names are used in back-reference  k lt name gt  in pattern and   name  in replacement string   Named capturing groups are still numbered with the same numbering scheme  so they can also be accessed via Matcher group int group    Internally  Java s implementation just maps from the name to the group number  Therefore  you cannot use the same name for 2 different capturing groups

User · Answer

For The Rest Of Us Here is a simple and clear example of how this works Regex    a-zA-Z0-9      s     a-zA-Z       s     0-9    String   quot    UserName10 John Smith 01123    quot  group 0   UserName10 John Smith 01123 group 1   UserName10 group 2     group 3   John Smith group 4     group 5   01123  As you can see  I have created FIVE groups which are each enclosed in parentheses  I included the    and    on either side to make it clearer  Note that none of those characters are in the RegEx and therefore will not be produced in the results  Group 0  merely gives you the entire matched string  all of my search criteria in one single line   Group 1 stops right before the first space because the space character was not included in the search criteria  Groups 2 and 4 are simply the white space  which in this case is literally a space character  but could also be a tab or a line feed etc  Group 3 includes the space because I put it in the search criteria     etc  Hope this makes sense

[java] Java regex capturing groups indexes

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