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;
}
}
}
##########################