Mastering Regex in Perl

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Mastering REGEX in Perl

What is Perl
Benefits of Perl
Advantages of using Perl scripting
Starting Perl by writing the first script,
Uses of Regular Expression,
grep functions
At the end of this module, you will be able to
Objectives

Hi there!
My name is Jose, I’m a computer
consultant, techie and trainer. Students
usually come to me and ask which computer
language they should use in their project
and why.
I’m here to help
Meet Mr. Jose

Hi There!
My name is Han, I’m Quality Analyst and my manager
asked me to automate the tasks. I’m confused which
language to use as I have tight deadlines and want to
make automation generic. I am here to meet Mr. Jose
and wanted to know which language should I use for
automation
Meet Mr. Han

Hi Jose, I work for investment
bank. My manager asked me to
automate all my tasks. On a
daily basis I interact with
millions of shares. I’m confused
which language should I use
Hi Han, seems you need to
interact with data and whenever
the huge data processing comes
to your mind Perl is the most
suitable computer language
Han is Confused!

Perl is one of the most popular open source interpreted programming language with a huge number of
programmers, libraries and resources
Perl has very powerful inbuilt regular expressions which often is the important reason when people decide
to use Perl for bulk text processing
Perl is platform independent and also used to generate html pages
Similar to Python, PHP but, with very powerful and flexible features
Inbuilt regular expression provides data filter and data transformation
Perl is nicknamed "the Swiss Army chainsaw of scripting language" due to its flexibility and power
What is Perl?

What are the Benefit of using Perl?
» Perl has relatively few keywords, simple structure, and a clearly defined syntaxEasy-to-learn
» Perl can run on a wide variety of hardware platforms and has the same interface on
all platforms
Portable
» Perl provides interfaces to all major commercial databases
» CPAN an archive of Perl library consist more than 20K modules
Databases
» One of Perl's greatest strengths is the bulk of the library is very portable and cross-
platform compatible on UNIX, Windows and Mac OS
Standard
Library
» Automatic memory management
» Automatic garbage collection
Memory
Management
» High-level data types and operations
» Object-oriented programming
» Easy Debugging Techniques
» Scalability
Others Benefits

About Perl
Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make
report processing easier. Since then, it has undergone many changes and revisions
Perl is not an official acronym but people say it is derived from Practical Extraction and Report Language
 As per the saying, frustrations of Unix shell programming led directly to the creation of Perl
It is an open source and interpreted language
Considered a scripting language, but is much more than that
Scalable, Object Oriented and Functional
Used by many Fortune 500 organizations
Simply, there is nothing which Perl cannot do

Less Restrictions
Developer Productivity
Program Portability
Support Libraries
Component Integration
Enjoyment
» Perl has the relatively less keywords and they are many ways to do the similar thing is a
philosophy of Perl
Why Perl?

Enjoyment
» Perl code is typically one-third to one-fifth the size of equivalent C++ or Java code. That
means there is less to type, less to debug, and less to maintain
Why Perl?

Enjoyment
» Perl programs run unchanged on all major computer platforms. For Example- Windows,
Linux, MAC OS etc.
Why Perl?

Enjoyment
» Perl comes with a large collection of prebuilt and portable functionality, known as the
Standard modules. These modules supports an array of application-level programming
tasks, from text pattern matching to network scripting
Why Perl?

Enjoyment
» Perl scripts can easily communicate with other parts of an application, using a variety of
integration mechanisms
Why Perl?

Enjoyment » Because of the ease of use and built-in toolset, Perl makes the programming more
pleasurable
Why Perl?

Users and Perl Projects
» Yahoo uses Perl in many of website development and data processing
» SpamAssassin is the well known SPAM filter software. It is part of
the Apache Software Foundation
» CiderWebmail, is an opersource product written in Perl and AJAX

Users and Perl Projects (Contd.)
» Twiki is one of the best-known wiki software with an orientation to
support companies. It is built primarily by the company with the same
name that also provides cloud-based hosted Twiki service
» Bugzilla is the well known bug-tracking system developed by and for
Mozilla. It is used in quite a lot of companies

Traditional Uses of Perl
Internet Scripting
System Utilities
Web ScrapingDatabase Programming Ad Targeting
Text Processing

Traditional Uses of Perl (Contd.)
Request
Result
NetworkCRM
LOB
ERP
ETL
Data
Warehouse
ETL Processing Network Programming

Write a First Program
 We can use any editor to create a scripts on Windows and vi editor on Linux
 The extension of the script is .pl
 Perl executable statements end with semicolons (;)
 Perl is case-sensitive
 Free form – whitespaces are ignored
 Comment begin with # (pound sign) – may be anywhere, not just beginning of line
 Perl also support multiline comment through POD (Plain Old Documentation)
 Using POD we can add the documentation in the scripts, these statements are not treated as executable statements
 __END__ is one of the special literal which is the logical end of the program

Write a First Program (Contd.)
 To execute the script, invoke the script using perl <script name>
 For LINUX users – you can execute the script while adding the she bang line (the interpreter address at the very first
line of the script) to make them self executable
Example
D:Edureka > perl helloWorldDemo.pl
D:Edureka > helloWorldDemo.pl

 Regular Expression is a set of characters together form the search pattern
 Main use of regular expression is to match patterns in any string forms
 The other use of regular expression ‘find and replace feature’
 Regular expression forms the generic pattern for the string matching with the help of pre-defined wildcard
characters
 Many language provide regular expression capabilities, some language have it inbuilt and other are having
regular expression libraries
 Regular expression is also known by regex or regexp
 In Perl regex is inbuilt, hence it is pretty good in performance
What is Regular Expression?

Real World – Regular Expression
I wish, if I could have the
software which filter all the
phone call starting with +140

Match Operator
We have match operator which matches the regex available in the string
 =~ (assignment operator followed by tilda operator is use for regex matching)
 !~ (Negation operator followed by tilda operator is use for regex un-matching)
~ operator after assignment
operator perform the regex
matching, REGEX are case
sensitive, m character in
matching regex is optional

A Word Match – Example
Output
Example

The First Wildcard
Wildcards (are also called as quantifiers) are the operator symbols which have specific meaning inside regular expression
For example: . (Dot or period) matches any character, digit, alphanumeric character except newline character (n).

Match Operator itself
In many cases, user may wants to match the operator symbol itself in the regular expression. We can suppress the wild
cards and special characters itself by backslash ()
Output
Example

Capturing and Grouping
Perl regex remember a group of strings which being the part parentheses in the regular expression
Inside regex, these groups are refer by back references. They are 1, 2,3 and so on..
Outside regex, these groups are refer by special variable $1, $2, $3 and so
These groups can also be fetched by variables assignment in list context called as capturing

Back References Example
Output
Example

Substitution
The another Perl operator that uses regular expressions allows us to provide find and replace feature
Regex are Greedy, means it will try to match as much it can!
This is called as substitution

Modifier ‘i’ and ‘g’
‘i’ modifier make the REGEX case insensitive
‘g’ modifier is for global search

Modifier ‘s’ and ‘m’
‘m’ modifier ^ and $ match more than once inside a string.
‘s’ modifier make . to match n as well

Modifier ‘x’
‘x’ modifier white spaces in the REGEX are ignored. This modifier is used for clean syntax

Greedy Property of REGEX Wildcards
Whenever Perl REGEX sees '*' or '+‘ or ‘?’ or {a,b} it will matches as much as it can
This property is greedy property of regex wildcards
Sometimes it’s an issue as substitute replace the matched string

Other Wildcards
These wildcard characters do not matches themselves. Until and unless they suppressed by backslash
Following are the other wildcards:
Wildcard Meaning
* matches Zero or more occurrence of previous character/s
+ matches One or more occurrence of previous character/s
? matches Zero or One occurrence of previous character/s

Wildcards Examples
REGEX Matches
AbC*
It matches A followed by b followed by either Zero or more occurrence of C. i.e. Ab, AbC,
AbCCCC, AbCCCCCCCCCCCC
AbC+
It matches A followed by b followed by minimum one or more occurrence of C i.e. AbC,
AbCCCCCCCC, AbCCC
AbC? It matches A followed by b followed by one or Zero occurrence of C. i.e. Ab, AbC
Ab(cd)*
It matches A followed by b followed by either Zero or more occurrence of cd i.e. Ab, Abcd,
Abcdcd
Ab(cd)+
It matches A followed by b followed by minimum one or more occurrence of cd i.e. Abcd,
Abcdcd
Ab(cd)? It matches A followed by b followed by either one or zero occurrence of cd i.e. Abcd, Ab

Combine Multiple Wildcards
REGEX Matches
Ab+C*
It matches A followed by minimum one or more occurrence of b followed by either Zero or more
occurrence of C. i.e. Ab, AbC, AbbCCC. AbbCCCCCCCCCC
A.C+
It matches A followed by any character followed by minimum one or more occurrence of C i.e. AZC,
AzCCC. AECCCCCCCCCCC
..C? It matches any two characters followed by b followed by one or Zero occurrence of C. i.e. Ab, AbC
<.*> It matches anything inside tags <> i.e. <HTML>, <TAGS>
( .+ ) It matches minimum one character inside brackets cd i.e. (Abcd), (a)
ab+c? It match a followed by one or more b followed by zero or one c. i.e. "abbbbc" or "abc", but not "ac"

Character Class in Regex
Character class is the set of any characters, digits or alphanumeric characters
While using the character class in Regex, it says any single character from the set
In character class we put a list of the characters in set inside square brackets like:
REGEX Matches
[abc] It matches any string which has either ‘a’ or ‘b’ or ‘c’
[abcdefghijklmnopqrstuvwxyz] It matches any string which has either ‘a’ or ‘b’ or ‘c’ or so on till ‘z’
[a-z] It matches any string which has either ‘a’ or ‘b’ or ‘c’ or so on till ‘z’
[0-9] It matches any string which has 0 or 1 or 2 or 3 till 9
[a-zA-Z0-9] It matches any string which has characters from a-z and A-Z and 0-9
[a-z_] It matches any string which has characters from a-z or _ (underscore)

Negate the Character Class
^ (carat) symbol inside character class is used to negate the character class in regex
 If we put the carat within the character class in Regex, it says none of the single character from the set
Here are few examples:
REGEX Matches
[âbc] It matches any string which has neither ‘a’ nor ‘b’ nor ‘c’
[âbcdefghijklmnopqrstuvwxyz] It matches any string which has neither ‘a ‘ or ‘b’ or ‘c’ or so on till ‘z’
[â-z] It matches the string which has neither ‘a’ or ‘b’ or ‘c’ or so on till ‘z’
[âeiou] It matches the string which has no vowels
[lL][âbc] It matches the string has ‘l’ or ‘L’ should not followed by ‘a’ nor ‘b’ nor ’c’
[â-z_] It matches the string doesn’t have a-z or _ (underscore)

Combine Character Class with Wildcards
REGEX Matches
[aA][0-9]+
It matches any string which has ‘a’ or ‘A’ followed by any number and occurrence can any
number of times
A+.[.?]
It matches any string which has ‘A’ any number of times followed by any character followed by
either ‘.’ or ‘?’
a[bc] It matches any string which has ‘a’ followed by either ‘b’ or ‘c’
A[abc]? It matches the string which has ‘A’ followed by zero or one occurrence of either ‘a’ or ‘b’ or ‘c’
[a-z_.]@ It matches the string has ‘a’ to ‘z’ or ‘_’ or ‘.’ followed by ‘@’

Character Class - Shortcuts
Character classes can also be represent by shortcuts
Following are the examples:
Shortcut Say Meaning
s Any space, tab or new line characters [ tn]
S Other than space, tab or newline character [^tn]
d Any digit [0-9]
D Other than digit [^0-9]
w Digits, characters or _ (underscore) [a-zA-Z0-9_]
W Other than digit, character or _ [^a-zA-Z0-9_]

Shortcuts with Wildcards
Shortcuts can also be used with wildcards
Following are the examples:
s+
Any number of space, tab or new line
characters
[ tn]+
S+
Other than space, tab or newline character
any number of times
[^tn]+
d+ Any digit any number of times [0-9]+
D+ Other than digit any number of times [^0-9]+
w+
Digits, characters or _ (underscore) any
number of times
[a-zA-Z0-9_]+
W+
Other than digit, character or _ any
number of times
[^a-zA-Z0-
9_]+

Meta Characters
^ ^ful Should start with a string i.e. matches ‘ful’ but not ‘wonderful’
$ ful$ Should be ended with a string i.e. matches ‘wonderful’ but not the word ‘Fultron’
{a,b}
Abc{1,2}
Abc{1}
It matches the string has ‘A’ followed by ‘B’ followed by minimum one occurrence if ‘c’
and maximum 2 occurrence.
(…) (w+) Grouping will be discuss in later slides
? Backspace – suppress the special meaning of quantifiers.
| Black|white Black or White in the string

Meta Symbols
A Aful Should start with a string i.e. matches ‘ful’ but not ‘wonderful’
Z FulZ Should be ended with a string i.e. matches ‘wonderful’ but not the word ‘Fulltron’
cA cA Match control A, cB Match control B
Q and E Q What is your name?E Quotes the meta characters till E (? Is question mark here not the quantifier)
b bfulb Looks for exact word ful
B BFulB Opposite of b
n and t t Match the n (new line) and t(tab character)

All in One Example
REGEX Matches
/full/ Matches ‘full’, ‘Wonderful’ and ‘Fultron’
/Ao+/ Matches Ao, Aoo, Aoooo
/A(oh)*/ Matches A, Aoh, Aohoh
/Yahoo{1,3}/ Matches Yahoo, Yahooo, Yahoooo
/Edurekas?/ Matches Edureka, Edurekas
/Checks+mates/ Matches Check followed by spaces followed mates
/$10/ Matches $10, $100, $101

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Mastering Regex in Perl

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