Perl Practical Extraction and Report Language

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Perl Practical Extraction and Report Language

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Title: Perl Practical Extraction and Report Language

1
Perl - Practical Extraction and Report Language

Perl is a programming language used in various
applications such as text processing. An
important feature of Perl is regular expression.
Useful sites for Perl www.cpan.org,
www.activestate.com, etc.
A simple, one-line Perl program/script
print "Hello World\n"
In Linux, add the following line at the top of
program file to tell the location of the Perl
interpreter and enable the warning option
!/usr/bin/perl -w
the symbol introduces a comment in Perl
set execution permission for the program file
chmod x hello
and execute it ./hello
In Windows, save the program file with the .pl
extension, and execute it with Perl interpreter
c\gtperl hello.pl
to enable warning option, add this line use
warnings

2
Scalar, _at_Array and Hash

Scalar, a number or a string or , starts with ,
e.g. name
Array, an array, starts with _at_, e.g. _at_names
Hash, an associative array, starts with , e.g.
names

scalar string "snm" num1 100 num2
1.01 print "The string is string\n" print "One
number is num1, another number is num2\n"
array and scalar myvar"my variable"
_at_myvar("one","two","three","four","five","six")
print "myvar\n" print value of a
scalar print "myvar1\n" print value of the
2nd element of _at_myvar print "_at_myvar\n" print
all elements of _at_myvar
3
Typing - use strict

Typing declaration of the type of variable may
not be used in short Perl programs such as the
example on previous page. Perl is referred as a
loosely typed language.
For longer Perl programs, it is better to add use
strict which requires that each variable be
declared before being used.

example of use strict use strict our (x, y)
split ' ', ltstdingt declare global variable
sub product my (num1, num2) _at__ declare
local variables return num1num2 rest of
the program Print product (x, y) \n
4
Print and Variable Interpolation

The print function is a list operator. It
accepts a list of things, separated by commas, to
print.
Double quote " " supports variable interpolation
Single quote ' ' does not support variable
interpolation.
string"Perl"
num10
print string, " and ", num
print "string and num\n"
print 'string and num\n'

5
Escaping

Escaping - the functions/meanings of special
characters such as , _at_ and are not taken.
Special characters are also called
metacharacters.
Escaping can turn some non-special characters
into something special.
A backslash \ is used to apply escaping, as in
the following example.
num1 10
print "\num1 is num1\n"
with \, the special meaning of is escaped,
the character is printed once,
followed by the string "num1 is " and the value
of num1
with \n, a new line is printed

6
Subroutines

A subroutine is a user-defined function
can be in the beginning, middle or end of a Perl
program
start with sub and then its name
a pair of curly bracket is required to
enclose the code
the area between the two brackets is called a
block
the prefix is used when calling a subroutine.
sub print_results print "\num is num\n"
num10
print_results call the subroutine
num
print_results call the subroutine

7
Subroutines passing return

To pass parameters to the subroutine, use ( ).
The parameters are passed through the list _at__
At the end of the subroutine, use return to
return the result.

sub product (num1, num2)_at__ receive two
values from main return num1num2 x 15
y 20 print product(x, y), "\n"
product(x, y) pass the values of x and y to
subroutine result returned by the subroutine is
then printed.
8
Testing 'truth' in Perl

What is 'true' in Perl
Any string is true except for "" and "0".
Any number, positive or negative, is true except
for 0.
Any undefined variable is false. An undefined
variable is one which doesn't have a value.
a 0 b"" c"0"
if (!a) print 'a is ',a," which is
false\n"
if (!b) print 'b is ',b," which is
false\n"
if (!c) print 'c is ',c," which is
false\n"
if (!d) print 'd is ',d," which is
false\n"

9
Comparison Operators

Comparison Numeric String
Equal eq
Not equal ! ne
Greater than gt gt
Less than lt lt
Greater than or equal to gt ge
Less than or equal to lt le
num115 num215
if (num1 num2) print "num1 equal num2.\n"
name1 John" name2 "May"
if (name1 name2) print "name1 equal
name2.\n"
to compare strings eq should be used instead of

10
Standard Input STDIN, chomp shift

To get input from user through keyboard, use the
ltSTDINgt function.
name ltSTDINgt
print "Your input is name\n"
The chomp function remove the trailing new line.
Compare the above example with the one below
nameltSTDINgt or, chomp(nameltSTDINgt)
chomp name
print "Your input is name\n"
Using shift
name shift
class shift
print "Your name is name,\tyour class is
class\n"
run this program with two arguments such as,
c\name.pl john 3A or ./name john 3A

11
Arrays

An array is an ordered list of scalar variables.
Each element in the array is indexed by a number
_at_names("Mary","Alex","John","Albert","Henry")
print _at_names no space between two elements
print "\nThe elements of \_at_names are _at_names\n"
print "The first element is names0 \n"
print "The first two elements are _at_names0,1\n"
print "The first three elements are
_at_names0..2\n"
print "The last element is names-1\n"
print "The index number of the last element is
names \n"
print 'There are ',scalar(_at_names)," elements in
the array\n"
An array can be initialized using "space" instead
of using comma
_at_namesqw(Mary Alex John Albert Henry)
quote words qw means splitting the list by words

12
Arrays push, pop, shift unshift

_at_names("Albert", "Alex", "Anna")
Add a new element
push _at_names, "Bob" Add to the end of array
print "_at_names\n" "Albert Alex Anna Bob" is
printed
unshift _at_names, "Betty" Add to the start of
array
print "_at_names\n" Betty Albert Alex Anna Bob
remove an element and return its value
name1 pop _at_names remove from the "end" of
array
name2 shift _at_names remove from the "start"
of array
print "name1 name2\n" "Bob Betty" is
printed

13
Arrays, For, Foreach, _

A simple for loop for accessing elements in an
array.
_at_names("Mary","Alex","John","Albert","Henry")
for (x0 x lt names x)
print "namesx\n"
The foreach function is used to loop through
arrays and hashes
foreach person (_at_names)
print "person"
elements in the array is accessed and printed
one by one
The default input _
foreach (_at_names)
print "_ " or just, print
if no variable is specified, _ is printed by
default

14
next last in a loop

Stopping a while loop that never ends.
while (1) this while loop never end!
print "press CTRL-C to stop!\n"
The last operator
_at_names('Mr Ho','Mrs Leung','Miss Lee','Dr
Wong','Ms Chan')
foreach person (_at_names)
last if person /Dr /
print "person\n"
after printing "Miss Lee" and detects "Dr ",
exits the loop
The next operator
_at_names('Mr Ho','Mrs Leung','Miss Lee','Dr
Wong','Ms Chan')
foreach person (_at_names)
next if person /Mr /
print "person\n"
the program skips "Mr Ho", then prints the rest

15
Hashes Associative Arrays
one way to define a hash countries('CH','China
','TH','Thailand','JA','Japan') another way
to define a hash countries(CHgt'China',THgt'Thai
land',JAgt'Japan') print "Enter the country
code " chomp (findltSTDINgt) print
"\nCountry code find\tCountry
countriesfind\n"

A hash is formed by pairs of key and value. Each
key is associated with a value.
Each key of a hash must be unique. If a key is
defined twice, the second value overwrites the
first.
The values pointed/indexed by different keys can
be the same.

16
_at_array and hash

In an array, elements/values are accessed with
their index number.
print "myarray1"
In a hash, each value is associated with an
unique string a key.
print "myhash'CH'"
To access an array element, uses an index number
and
To access a hash element, uses a key and

17
_at_array or hash

Using hashes
When you want to look something up by a keyword,
for example, a program returns an IP address when
given a host name.
To relate one set of data (keys) to another set
of data (values).
Using arrays
Use an array when you need an ordered list of
elements. The elements in an array always stay
in the same order when they are created.
Unlike an array, a hash is not an "ordered" list.

countries(CHgt'China',THgt'Thailand',JAgt'Japan'
,HKgt'Hong Kong') print countries the print
results show no recognizable sequence
18
Accessing Hash Elements

Assign/add a pair of key and value
countriesHK'Hong Kong'
Remove a pair of key and value
delete countriesHK
Print all keys
print keys countries
Print all values
print values countries
Print a slice of the hash
print _at_countriesCH,TH
Print how many elements in the hash
print scalar(keys countries)

19
Hash each, foreach

countries(CHgt'China',THgt'Thailand',JAgt'Japan'
,HKgt'Hong Kong')
while ((code, name) each countries)
print "The key code points to name\n"
call each pair of key and value
foreach code (keys countries)
print "The key code points to
countriescode\n"
a "temporary" variable code is used to store
the key
foreach (keys countries)
print "The key _ points to countries_\n"
if a variable is not specified, the default,
_ is used

20
Hash sorting

countries(CHgt'China',THgt'Thailand',JAgt'Japan'
,HKgt'Hong Kong')
sorting based on the keys (strings)
hash elements are accessed in the order the
keys are sorted
a simple sort
foreach (sort keys countries)
print "The key _ points to countries_\n"
a reverse sort
foreach (reverse sort keys countries)
print "The key _ points to countries_\n"

21
Regular Expressions (regex)

What is a regular expression? A regular
expression is simply a string that describes a
pattern. For example, the patterns typed into a
search engine the patterns used to list files in
a directory (dir .exe)
In Perl, the patterns described by regular
expressions are used to search strings, extract
parts of strings, search and replace, etc. For
example,
if ("Hello World" /World/) print "it
matches\n"
/World/ tells Perl to search for the pattern
enclosed by the two //, in this case, the word
World.
The operator associates the string with the
regex and produces a true value if the regex
matched.

22
Examples of regex
_"perl for System Administration" the
string to match with if (_ /perl/) print
"1 Found perl\n" if (/perl/) print "2
Found perl\n" same as above, not used if
(/PeRl/) print "3 Found PeRl\n" fail, case
sensitive if (/PeRl/i) print "4 Found PeRl\n"
with /i, not case sensitive print "5
Found!\n" if / / "if" can be put after
"print" if the regex / / find a space in _,
print the string "5 Found!" print "6 Found!\n"
unless _ ! / / 'unless' '!' are both
negative find" for " create some variables
to search for if (/find/) print "8 Found find
in _\n"
23
Character Class

The square brackets enclose single character to
be matched.
To negate the entire regex, change to ! , for
example,
(_ ! /KCarel/)
To negate specific character, use caret , for
example,
/KCarel/
match K or C, then ar, then anything EXCEPT e
or l

_at_namesqw(Karl Care Karen Card) foreach
(_at_names) if (/KCarel/) print "_ is
matched!\n" matches K or C, then ar, then e
or l
24
More examples of regex matching

Start with use , e.g., /n/ matches those
strings that begin with n.
End with use , e.g., /s/ matches those strings
that end with s
Matching x times
Zero or more times use
One or more times use
Zero or one time use ?
Exactly n times use n
At least n times use n,
At least n but not more than m times use n,m

25
Return the match

Regex can return what is found to a specified
variable.
star has no value assigned because 0-9 match
0 or more characters from 0 to 9 at the very
start of the regex
plus get the 2200 value because 0-9 match one
or more characters from 0 to 9. If 0 to 9 is not
found the match will fail. Once a 0-9 is found,
the match continues as long as the next character
is 0-9, then it stops.

_'This year is 2009'(one)/(0-9)/
return one digit only (star)/(0-9)/
match zero or more character (plus)/(0-9)/
match one or more character print
"\oneone \starstar \plusplus\n"
print result is one2 star plus2009
26
Return the match 1, 2

With parenthesis ( ) around the regex, the first
match is put into a variable called 1.
In the example below, the regex begin matching
for lt. When lt has been found, the first
parenthesis ( ) start to match for staff and put
the value into the variable 1 when matched.
The regex then continues matching for the
character _at_ . When _at_ is found, the 2nd
parenthesis ( ) starts to match for vtc.edu.hk
and put the value into the variable 2 when
matched.

_'My email address is ltstaff_at_vtc.edu.hkgt.'
/lt(staff)\_at_(vtc.edu.hk)gt/ print "1 at 2 \n"
print result staff at vtc.edu.hk
27
Match return the unkown

The last example requires that the email address
be know in advanced.
In the example below, the regex does not know
what to match for.
In the /(.)/ regex, the dot (or period) match
for any character, and the match for zero or
more characters.
In the regex /lt(.)gt/, a match start with lt and
end with gt
The regex returns any characters between lt and gt
into 1

_'My email address ltstaff_at_vtc.edu.hkgt.' print
"1 \n" if /(.)/ result My email address
ltstaff_at_vtc.edu.hkgt. print "1 \n" if
/lt(.)gt/ result staff_at_vtc.edu.hk
28
Substitutions and Split

Search and replace examples
_ "your home , my home, sweet home"
s/home/apple/ replace once
print _,"\n"
s/home/apple/g replace many
print _,"\n"
Using split
_at_fields split //, "nameagetelephone"
_at_fields split /,/, "name,age,telephone"

29
Calling external commands/processes

There are several ways for Perl to start external
commands/processes, such as system and
backticks/backquotes
The system function
It calls and runs an external commands/processes,
e.g.,
system("dir")
Backticks/backquotes can start an external
commands/processes
Note that the single quote '' is different from
the backquote
dirdir
print dir

30
Using system

Using the external command system together
external commands such as the Windows net use
and net user commands, you can perform various
system administration works, for example,
system("net user abc 123 /add")
create a user account abc with password 123
system("net use drive_letter \\\\xxx\\yyy 123
/userabc")
drive_letter such as z is the drive that you
want to map
where xxx is the server name or ip address
yyy is the shared folder name
abc is username and 123 is the password

31
Opening Files

Assume you have a file called data.txt in the c\
directory, the following program can open the
file and read the content.
If the open operation fails, the code next to or
is evaluated.
The code die means exiting the program after
printing the specified message.
The ! provides the error message.
The special variable . is the current line
number, starting at 1.
The special variable _ is the default variable.
If there is not an user-defined variable, _ is
assigned the content read from the input file.

data"c\\data.txt" open DATA, data or die
"Cannot open data to read !" while (ltDATAgt)
print "Line . is _"
32
Opening Files

Variables which represent files are called "file
handles" which does not begin with a special
character. For example, the file handle DATA
used in the example shown in previous slide.
The input operator, the angle brackets ltgt, reads
from the beginning of the line until and
including the first newline. It returns one line
from standard input.
The data read by the ltgt operator goes into _
On the next iteration of the loop, data is read
from where the last read left off, up to the next
newline. And so on until there is no more data
in the file. When that happens the condition is
false and the loop terminates.
The ltgt operator returns undef when there is no
more input.

33
Writing appending data to a File

To open a file for writing data, add gt to the
filename.
To open a file for appending data, add gtgt to the
filename.
When print to a file, just specify the name of
file handle.
Closing a file after use is not mandatory unless
you wish to open another file.
Note that localtime holds the value of current
time.

out"out.txt" open OUT, "gtout" or die "Cannot
open out for write !" for i (1..10)
time localtime print OUT "i The time is
now ", time,"\n" close OUT
34
Directory and File Tests

To check if the file (filename) exists in the
current directory
print "file exists\n" if e filename
To check how long since the file has been
modified
age -M filename
To get the size of a file
size -s filename

Test Meaning -r readable -w writable -s size
(bytes) -f a file -d a directory -M modificatio
n age (days) -A access age (days)
35
Directory File Operations

Moving around (change directory) the directory
tree
chdir "/etc"
Globbing produces a list of files/sub-directories
in current directory
_at_files glob "" get names of files
sub-directories
_at_pl_files glob ".pl"
Making and removing directories
mkdir "temp", or warn "cannot make dir !"
rmdir "temp" remove empty directory only
Renaming files
rename "old.txt", "new.txt"

36
Directory Handle readdir()

Opening a directory is similar to opening a file.
The directory handle can be passed to the
function readdir( ) to get a list of all of the
files and sub-directories.

chdir "c\\Windows" opendir DIR,"." or die
"Can't open the directory !\n" _at_names
readdir(DIR) foreach name (_at_names) if (-d
name) print "found a directory name\n"
print the name if it is a directory. if (-f
name) print "found a file name\n" print
the name if it is a file. closedir(DIR)
37
Referencing Data

References indirectly refer to other data
References are like pointers, but done right
Backslash operator creates a reference
References are scalars

my _at_fruit qw(apple banana cherry)
my fruitref \_at_fruit

38
Dereferencing Data

Dereferencing yields the data
Appropriate symbol dereferences original data
Arrow operator (-gt) dereferences items

my _at_fruit qw(apple banana cherry)
my fruitref \_at_fruit
print "I have these fruits _at_fruitref.\n"
print "I want a fruitref-gt1.\n"
I have these fruits apple banana cherry.
I want a banana.

39
Anonymous Data

References allow you to create anonymous data
Referencing arrays and hashes are common
Build unnamed arrays with brackets ( )
Build unnamed hashes with braces ( )

my fruits "apple", "bananas", "cherries"
my wheels unicycle gt 1,
bike gt 2,
tricycle gt 3,
car gt 4,
semi gt 18
print "A car has wheels-gtcar wheels.\n"
A car has 4 wheels.

40
Perl Modules

Perl modules that can be added to a standard Perl
installation, for example, the NetLDAP module
for directory (LDAP) applications and the module
NetSNMP for network management applications.
Having installed a module the documentation is
available using the standard "perldoc" command,
e.g. perldoc NetLDAP.
Two basic types of module Function and Object.
Functional modules export new subroutines and
variables into your program.
Using object type modules create an object,
access methods through the object
The NetLDAP and NetSNMP modules are examples
of object type module.

41
Perl Objects

Perl objects are special references that come
bundled with subroutines (called "methods") and
data.
To use Perl objects, first declare to use a
object type module, such as ,
use NETLDAP
Then instantiate an object (create an instance),
such as,
ldapNetLDAP-gtnew("localhost",portgt389,versi
ongt3)
Refer to the examples shown in next slide for how
to use methods via the object.
ldap is object of class NetLDAP
Interact with objects through object methods like
print
Notice that the arrow -gt is used to invoke a
method.

42
NetLDAP module session bind
!/usr/bin/perl -w use NetLDAP ldap
NetLDAP-gtnew("localhost", portgt389,
versiongt3) or die "cannot make session,
!\n" Anonymous bind resultldap-gtbind()
resultldap-gtbind("cnict,dccsa-snm,dccom",
passwordgt"snm") errorresult-gterror() if
(result-gtcode()) print "cannot bind
successfully, error\n" exit 1
43
NetLDAP module - search

mesg ldap-gtsearch(
base gt "dccsa-snm,dccom",
scope gt "sub",
filter gt "(objectClassperson)",
attrs gt "cn", "sn"
to read all attributes attrs gt ""
)
foreach myentry (mesg-gtall_entries())
myentry-gtdump() display all attributes
tempmyentry-gtget_value("cn")
print temp get and display an attribute
ldap-gtunbind()

44
NetLDAP module add, modify, delete

refer to page 46 for the creation of the object
ldap
dn "cnjohn,oustaff,dccsa-snm,dccom"
Add an entry
result ldap-gtadd( dn,
attrs gt cn gt 'john',
objectClass gt 'person',
sn gt 'ho')
Modify an entry by adding an attribute value
result ldap-gtmodify( dn,
add gt description gt "hi")
Modify an entry by replacing an attribute value
result ldap-gtmodify( dn,
replace gt description gt "hi, there")
result ldap-gtdelete(dn) Delete an entry

45
Using NetSNMP module

The modules NetSNMP, NetSNMPHostInfo and
NetSNMPInterfaces provide an OO interface to
SNMP.
One NetSNMP object corresponds to one remote
SNMP agent or manager.
Methods that require a response return a hash
reference containing the query results. Keys in
the hash are dotted OIDs.
Returns undefined value on failure. The error()
method shows cause of failure.
Method parameters uses a dashed-option naming
style
object-gtmethod( -argument gt value )

46
NetSNMP Session()