Perl Learning - 12 (Perl RE)

发布时间：2020-12-15 21:02:23 所属栏目：大数据来源：网络整理

导读：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 unmat

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(....) d1){
?print "It matched the same after y and d!n";
}

$_="yabba abba doo";
if(/y(.)(.)21/){
?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
[ftnr ] 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"
[dD] matches any numberic or non-numberic,that's any character including "n"
[^dD] 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;
?}
}

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

#!/usr/bin/perl

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

（编辑：李大同）

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