Perl Learning - 12 (Perl RE)

Another good feature of Perl is RE,PerlRE.

In Perl,RE usually means patten,a match (or unmatch) of some characters template.

The patten can divide any characters into two parts: match and unmatch. All RE does is to determine match or unmatch to a certain characters.

 

$_="yabba dabba doo";
if(/abba/){
 print "It matched!\n";
}

 

Expression /abba/ tries to find 'abba' from $_,if found then true otherwise false.

All skills used in " " can be used in RE patten.

 

There are some magic characters do magic match in RE.

 

* is 0 or more times of character before it.
+ is 1 or more times of character before it.
? is o or 1 times of character before it.

 

a* means 0 or more 'a',like a aa aaaaaa or nothing(0 times).
a+ means 1 or more 'a',like a aa aaaaa.
a? means 0 or 1 'a',that's a or thing.

 

() can make group in patten.

 

/fred+/ matches 'fre' with 1 or more 'd',like fred fredd fredddddd
/(fred)+/ matches 1 or more 'fred',like fred fredfred fredfredfredfred

 

() can also catch the patten and put in \1 \2 \3 ... accordingly.

 

$_="abba";
if(/(.)\1){ # matched bb
 print "It matched same character next to itself!\n";
}

 

$_="yabba dabba doo";
if(/y(....) d\1){
 print "It matched the same after y and d!\n";
}

 

$_="yabba abba doo";
if(/y(.)(.)\2\1/){
 print "It matched the same after y and d!\n";
}

 

If there are () in (),we count the left ( refer to the number \1 \2 \3 ......

 

(  # The first one,\1
 (.) # The second,\2
 (.) # The third,\3
 \3
 \2
)

 

$_="aa11bb";
if(/(.)\111/){
 print "It matched!\n";
}

 

Perl tries to explain \111,which fails. In Perl 5.10,we can use \g{1} to catch \1

 

$_="aa11bb";
if(/(.)\g{1}11/){
 print "It matched!\n";
}

 

/fred|barney|betty/ matches fred OR barney OR betty,if one of them exists,then matches.

 

/fred( |\t)+barney/ matches fred and barney with one or more space to tab in them.

 

/fred (and|or) barney/ matches 'fred and barney' OR 'fred or barney'.
/fred and barney|fred or barney/ matches the same as above.

 

[] matches ONE of the characters in it.

 

[abcwxyz] matches anyone of the 7 characters 'abcwxyz'
[^ab] matches any character but not 'a' or 'b'
[abcwxyz] is same as [a-cw-z],[a-zA-Z] matches all 52 English characters.

[0-9] matches all numberic characters.

[0-9] cab be written as \d
[A-Za-z0-9_] can be written as \w
[\f\t\n\r ] can be written as \s

 

[^\d] can be written as \D
[^\w] can be written as \W
[^\s] can be written as \S

 

. matches any character but not "\n"
[\d\D] matches any numberic or non-numberic,that's any character including "\n"
[^\d\D] matches nothing at all!

 

Exercises:

 

1. Write a program,read data from input,print the line when meets 'fred'.

 

#!/usr/bin/perl

while(<>){
 if(/fred/){
  print;
 }
}
##########################

 

2. Modify above program,also print the line with Fred in it.

 

#!/usr/bin/perl

while(<>){
 if(/[Ff]red/){
  print;
 }
}
##########################

 

3. Write a program,print only if the input line has a '.'

 

#!/usr/bin/perl

while(<>){
 if(/\./){
  print;
 }
}
##########################

 

4. Write a program,print the lines that has words with first char upper case.

 

!/usr/bin/perl

while(<>){
 if(/[A-Z][a-z]+/){
  print;
 }
}
##########################

 

5. Write a program,print the line with a non-space character stands besides itself.

 

#!/usr/bin/perl

while(<>){
 if(/(\S)\g{1}/){
  print;
 }
}
##########################

 

6. Write a program,print the lines with both wilma and fred.

 

#!/usr/bin/perl

while(<>){
 if(/fred.*wilma|wilma.*fred/){
  print;
 }
}

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

 

OR

 

#!/usr/bin/perl

while(<>){ if(/fred/){  if(/wilma/){   print;   }  } }##########################