What does 1 mean in Perl

Question

What does  1 mean in Perl  Further  what does  2 mean  How many  number variables are there

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In general  questions regarding  magic  variables in Perl can be answered by looking in the Perl predefined variables documentation a la   perldoc perlvar   However  when you search this documentation for  1  etc   you ll find references in a number of places except the section on these  digit  variables  You have to search for    lt digits gt    I would have added this to Brian s answer either by commenting or editing  but I don t have enough rep  If someone adds this I ll remove this answer

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The variables  1     9 are also read only variables so you can t implicitly assign a value to them       1    foo   print  1    That will return an error  Modification of a read-only value attempted at script line 1   You also can t use numbers for the beginning of variable names       1foo    foo   print  1foo    The above will also return an error

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I would suspect that there can be as many as 2  32 -1 numbered match variables  on a 32-bit compiled Perl binary

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These are called  match variables   As previously mentioned they contain the text from your last regular expression match   More information is in Essential Perl   Ctrl   F for  Match Variables  to find the corresponding section

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1   2  etc will contain the value of captures from the last successful match - it s important to check whether the match succeeded before accessing them  i e    if    var    m            use  1 etc        An example of the problem -  1 contains  Quick  in both print statements below      usr bin perl   Quick brown fox     m    quick    ix  print  Found   1 n     Lazy dog     m    quick    ix  print  Found   1 n

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Since you asked about the capture groups  you might want to know about    too     Pretty useful     use Data  Dumper   text    hiabc ihabc ads byexx eybxx   while   text      hi ih abc  bye eyb xx igs        print Dumper            OUTPUT     VAR1    hi      VAR1    ih      VAR1    bye      VAR1    eyb

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The  number variables contain the parts of the string that matched the capture groups         in the pattern for your last regex match if the match was successful   For example  take the following string    text    the quick brown fox jumps over the lazy dog      After the statement   text    m   b           1 equals the text  brown

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The number variables are the matches from the last successful match or substitution operator you applied   my  string    abcdefghi    if   string      abc def ghi          print  I found  1 and  2 n       Always test that the match or substitution was successful before using  1 and so on  Otherwise  you might pick up the leftovers from another operation   Perl regular expressions are documented in perlre

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As others have pointed out  the  x are capture variables for regular expressions  allowing you to reference sections of a matched pattern   Perl also supports named captures which might be easier for humans to remember in some cases   Given input  111 222     d   s   d       1 is 111   2 is 222  One could also say       lt myvara gt  d   s    lt myvarb gt  d         myvara  is 111     myvarb  is 222

[regex] What does $1 mean in Perl?

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