Perl Learning - 14 (Catch var, (:?), (:), \g{LABLE}, {m,n}

Variable can be inserted in /patten/, similar to "$variable"

 

#!/usr/bin/perl
my $what="fred";

while(<>){
 if(/^($what)/){
 print "We saw $what in beginning of $_";
 }
}

 

() can ctach the characters in it as varables, called $1, $2, $3 ....

The first () gets $1, second gets $2, .... caculate by the left (

 

$_="Hello there, neighbor";
if(/\s(\w+),/){
 print "the word was $1\n"; # print "the word was there"
}

 

$_="Hello there, neighbor";
if(/(\S+) (\S+), (\S+)/){
 print "words were $1 $2 $3\n";
}

 

The life sycle of a caught variable go until next successful match, which overide it.

If we only want () make group but not catch variable, we can use non-catch ().

 

$_="a saurus steak for dinner";
if(/(?:bronto)?saurus (steak|burger)/){
 print "Fred wants a $1\n";
}

 

The ?: behind ( makes that () not catch variable.

 

We can name the caught variable instead of $1 $2 .... the numberic style.

 

my $names="Fred or Barney";
if($names=~/(?<name1>\w+) (?:and|or) (?<name2>\w+)/){
 print "I saw $+{name1} and $+{name2}\n";
}

 

(?<LABLE>PATTEN) LABLE can be any name we like, all the names are stored in hash %+

 

Using the named variables we can insert more () not worrying the $number, \g{1} can also named as \g{lable}

 

my $names="Fred Flintstone and Wilma Flintstone";
if($names=~/(?<last_name>\w+) and \w+ \g{last_name}/){
 print "I saw $+{last_name}\n"; # Flintstone
}

 

if("Hello there, neighbor"=~/\s(\w+),/){
 print "That actually matched '$&'.\n";
}

 

if("Hello there, neighbor"=~/\s(\w+),/){
 print "That was ($`) ($&) ($').\n";
}

 

$& is what /patten/ matches, $` is parts before $&, $' is parts after $&.

 

/a{m,n}/ matches 'a' appears m to n times
/a{m,}/ matches 'a' appears at least m times
/a{m}/ matches 'a' appears exactly m times

 

RE has its connect order like this:

 

Patten  Examples
()   (...) (?:...) (?<LABLE>...)
* + ? {} a* a+ a? a{m,n}
^ $   abc ^a  a$
|   a|b|c
element  a  [abc] \d  \1

 

Exersices:

 

1. Write a program, make it match 'match' and print the parts before and after it.

 

#!/usr/bin/perl

while(<>){
 if(/match/){
  print "That was |$`<$&>$'|\n";
 }
}
##########################

 

2. Write a program, make it match words(made of \w) end with 'a'.

 

#!/usr/bin/perl

while(<>){
 chomp;
 if(/\w+a\b/){
  print "That was |$`<$&>$'|\n";
 }
}
##########################

 

3. Modify last program, catch the words end with 'a' in $1. And print it like: $1 contains 'Wilma'.

 

#!/usr/bin/perl

while(<>){
 chomp;
 if(/(\w+a\b)/){
  print "\$1 contains '$1'\n";
 }
}
##########################

 

4. Modify last program, catch words into named variable. And print it like: 'word' contains 'Wilma'.

 

#!/usr/bin/perl

while(<>){
 chomp;
 $lable="word";
 if(/(?<$lable>\w+a\b)/){
  print "'$lable' contains '$+{$lable}'\n";
 }
}
##########################

 

5. Modify last program,  print the word ended with 'a' and the following 5 characters(if there are).

 

#!/usr/bin/perl

while(<>){
 chomp;
 if(/(?<word>\w+a\b)(?<more>.{0,5})/){
  print "$+{word}|$+{more}\n";
 }
}

##########################

 

6. Write a new program, print all lines ended with sapce character(s), print more character(s) to make it human readable.

 

#!/usr/bin/perl

while(<>){
 chomp;
 if(/\s+$/){
  print "$_|\n";
 }
}
##########################