Regular expressions (regex) are patterns used to match character combinations in strings. This document introduces regex syntax and shows how to master regex from simple to complex patterns. It explains that regex can be used to avoid writing extensive code by concisely matching strings, and is supported by programming languages, tools like log analyzers, and databases. The document guides the reader through the regex syntax with examples to practice and eventually write their own complex regex patterns.

1. Regular Expressions From Beginner To Expert By Varun
Date: December 24, 2018
In this document want to show how to master regex. I wasn’t good at regex until I realized its power and
the necessity to understand and master it.
---- Avoid writing lines and lines of code by using power of regex.
---- The regex shown here will take you from a simple example to the complex patterns.
---- Lot of tools apart from all the programming languages support regex.
---- I personally know integration tools such as Ca-Api-Gateway, Apigee, TibcoFlogo, Mulesoft where
regex can be used and match complex patterns, save unnecessary code for pattern match.
---- Logging slutions like ELK, Splunk support full on regex.
---- Analytical tools such as Tableau support full on regex.
---- Please follow ths document and practice along by the end of it. You will start writing complex regex
on your own.

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Contents
Introduction ......................................................................................................................................3
What are regular expressions.....................................................................................................3
Why to learn regular expressions ..............................................................................................3
How regex is created ...................................................................................................................3
Tools Used....................................................................................................................................4
Basic Syntax Regex .....................................................................................................................5
All About Characters .......................................................................................................................5
Character Literals.........................................................................................................................5
Character Classes........................................................................................................................6
Forward Boundary .......................................................................................................................7
Character Range...........................................................................................................................8
Regex Live Example ..................................................................................................................10
Negation Characters ..................................................................................................................12
Meta Characters .............................................................................................................................13
What are Meta Characters .........................................................................................................13
Wild Card Meta Characters Part 1 ............................................................................................15
Escaping Meta Characters ........................................................................................................19
Predifined Character Classes ...................................................................................................20
Anchors And Word Boundary.......................................................................................................22
Anchors.......................................................................................................................................22
Word Boundary ..........................................................................................................................24
Quantifiers ......................................................................................................................................26
? quantifier..................................................................................................................................26
* Quantifier..................................................................................................................................28
Matching Example......................................................................................................................28
+ Quantifier .................................................................................................................................32
{} Quantifiers To Limit Range ...................................................................................................33
{min,} Quantifiers .......................................................................................................................33
{min,max} Quantifiers................................................................................................................34
Greedy Quantifiers.....................................................................................................................35

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Lazy Or Reluctant Quantifiers...................................................................................................36
Alternative For Lazy Quantifiers...........................................................................................36
Greedy Quantifiers vs Lazy Quantifiers...............................................................................38
Increasing the performance of the RegeX...........................................................................39
Groups In Regex ............................................................................................................................40
Group Examples.........................................................................................................................40
Back Refrencing In Groups.......................................................................................................42
Non-Capturing Groups ..............................................................................................................43
Alteration In Groups ..................................................................................................................43
Nesting in Groups/Alternation ..................................................................................................44
Assertions.......................................................................................................................................45
Look Ahead Assertion ...............................................................................................................45
Positive Look Ahead Assertion ................................................................................................45
Negative Look Ahead Assertion ...............................................................................................46
Look Behind Assertions............................................................................................................46
Positive Look Behind Assertion...............................................................................................46
Negative Look Behind Assertion..............................................................................................47
Real Life Scenarios ........................................................................................................................47
Example From My Project Work ...................................................................................................49

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Introduction
What are regular expressions
Why to learn regular expressions
Used in all programming languages. Also used in caApiGateway and Apigee
For example validating user input into a form
How regex is created

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Tools Used
Tools used for learning the regex during the course of this document.
Online regex tester
https://regex101.com
https://regexr.com
https://www/regextester.com
Offline Regex Testers
https://atom.io
reach regex by using ctrl+F or below steps

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Regexp are case sensitive.
Basic Syntax Regex
Used in all programming languages. Also used in Api Management tools such as, caApiGateway and Apigee
All About Characters
Character Literals
Takes the string as it is.

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Regexby default is case sensitive.
:/Java/gm (Case sensitive)
:/Java/gmi (Case insensitive)
Regex starts searching from left to right
Character Classes
Character class are enclosed in [] brackets

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Forward Boundary
>> Forward boundary uses the following pattern ‘b’
say in this you want to match only the word bet and not sub string that may contain bet.

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Character Range

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Say there are 5 employes and you want to match only the emp 1,2,3
Write a regex to match the first 3 words.
Indexes of alphabets and numbers

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You needs to match the index when defining the character ranges you need to go from small to big
[A-z] This is correct
[a-Z] This is wrong.
Regex Live Example
<!DOCTYPE html>
<html>
<head>
<title>Testing Regular Expressions in HTML forms</title>
</head>
<body>
<h1>Test your Regular Expressions here</h1>
<fieldset>
<legend>Write your Regular expressions</legend>
<form>
<div>
<label for="display-name"> Prove that you are not a robot:
<span class="warning">*(Enter a single English capital letter.)</span>
</label>
<input type="text" id="display-name" name="ip-display"
pattern="[A-Z]"
title="Enter any single English capital Letter only"/>
</div>
<div>
<input type="submit" class="submit" value="Submit" />
</div>
</form>
</fieldset>
</body>
</html>

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Negation Characters
Are represented with a ^ symbol inside [] brackets.
[^a-e] will match all characters except a-e
Example 1:-
 The First Character should be any character except a comma(,)
 The Second Character must be a comma(,)
 The Third Character should be any character except a comma(,)
Example 2:-
 The First Character must be a vowel(a,e,i,o,u)
 The Second Character must be a English alphabet(it can be lowercase or
uppercase English alphabets)

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 The Third Character must a single number
 The Fourth Character should be any character except a Hyphen(-)
 The Fifth Character must be a Hyphen(-)
 The Sixth Character should be any character except a Hyphen(-)
Meta Characters
What are Meta Characters
In regex each character is described as normal character or meta character.
Special character that have special meaning.

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used to preceed a meta char or predefine character class.
^ negation character and used in anchors
^ Is also used for matching a boundary at the start of the line
$ Is used for matching a boundary at the end of the line
. used to match any character aprart from new line
? quantifiers
* quantifiers
+ quantifiers
{} quantifiers
| Alternation
() Grouping
[] Ranges
= Assertions (look ahead, look behind)
! Assertions (look ahead, look behind)

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Wild Card Meta Characters Part 1
. meta character is also called wild card metacharacter
To match only one character

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New line char is represented by n. Line break.

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s whitespace
Matches new line followed by whitespace
b is a boundary between a word and a non word character

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Escaping Meta Characters
Meta characters are escaped by

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Predifined Character Classes
To capture a 3 digit number within “”

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Anchors And Word Boundary
Anchors
Are used match before the beginning or end of
1) String
2) Line

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Word Boundary
Is used to match the boundary between word and non-word character.

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Captures all word boundaries
All non-word boundary characters.

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Quantifiers
In regx there are 4 types of Quantifiers.
A quantifier is used after a charcter or group and decides how the character or group before the quantifier
will occur.
? quantifier

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* Quantifier
Matching Example
Q.1) Create a Regular Expression to match a pattern like below -

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User has to provide a valid Name of company in an on line Form and it can have
any alphanumeric value.
Hint:
-----
Name of a company can contain alphanumeric characters
Assumption:
-----------
Here, we assume that the names of the company does not contain any other
special characters. If your requirement is to match a company name with special
characters as well, then the RegeX will change accordingly.
Different waves to achieve it.

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<!DOCTYPE html>
<html>
<head>
<title>Testing Regular Expressions in HTML forms</title>
</head>
<body>
<h1>Test your Regular Expressions here</h1>
<fieldset>
<legend>Write your Regular expressions</legend>
<form>
<div>
<label for="display-name"> Enter a company name here:
<span class="warning">*(Enter an alpha numeric character.)</span>
</label>
<input type="text" id="display-name" name="ip-display"
pattern="^[0-z][A-zds]*$"
title="Special Characters Not Allowed"/>
</div>
<div>
<input type="submit" class="submit" value="Submit" />
</div>
</form>
</fieldset>
</body>
</html>
Command explanation.

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+ Quantifier

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{} Quantifiers To Limit Range
{min,} Quantifiers

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{min,max} Quantifiers

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Greedy Quantifiers
There is a fix for the greedy natures of the quantifiers by making the quantifiers “Lazy” or “Reluctant”
quantifiers.

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Lazy Or Reluctant Quantifiers
Alternative For Lazy Quantifiers
When making a quantifier Lazy this will result in increased processing time and this when used in
applications(bulding live applications) will cause lags. This can be over come by using negation characters.

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Greedy Quantifiers vs Lazy Quantifiers

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In the pattern match of xml tags we have seen earlier the Lazy quantifiers are much better than greedy
quantifiers.
Increasing the performance of the RegeX
The normal behavior of a quantifier is always Greedy and in some cases, Greedy quantifiers can lead to performance issues and in
some cases, Lazy Quantifier can also lead to performance issues.
Note:
If we find that Greedy Quantifiers have performance issue in your search pattern, then try changing it to Lazy Quantifier and see if it
improves the performance. On the other hand, if you have a lazy Quantifier which has a performance issue, try to change it into Greedy
Quantifier to see if the performance increases. We can also try to use the Negation character to improve the performance.
The key to performance is to always remember -

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** Greedy matches the longest possible string
** Lazy matches the smallest possible string
So, remember this, if we try to make Greedy quantifier as Lazy then the meaning will change like this -
{min,max}? - Repeat minimum 'min' times and maximum 'max' times, but as few times as possible(lowest is 'min' times)
{min,}? - Repeat minimum 'min' times and maximum any times, but as few times as possible(lowest is 'min' times)
*? - Repeat any number of times, but as few times as possible(lowest is 0 time)
+? - Repeat any number of times, but as few times as possible(lowest is 1 time)
?? - Repeat either 0 time or 1 time, but as few times as possible(lowest is 0 time)
Groups In Regex
1) Improves the redability of regex
2) Groups can be reused in complex expressions.
Group Examples

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The above regex is to match word regex and match only 3 numbers after it.

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Back Refrencing In Groups
In order to match all the words in the list

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Non-Capturing Groups
In order to optimize a regex it is better to make a group non-capturing group it optimizes the regex and
increases the performance.
A group can be made non-capturing by using the following syntax (:?)
Alteration In Groups

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Nesting in Groups/Alternation

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Assertions
Look Ahead Assertion
Positive Look Ahead Assertion

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Negative Look Ahead Assertion
Look Behind Assertions
Positive Look Behind Assertion

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Negative Look Behind Assertion
Real Life Scenarios
index.html style.css validation.js
Matching First/Last Name
Rules to be followed for First Last name

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Modify the first/last name as shown below
Matching Username
Modify the .js as shown
Matching Passwords

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Matching Email
Matching Phone Number
Example From Real World
After learning what I learnt from the start till this point in this document was able to do complex regex directly
in the real world.
Payload
{
"shipments": [
{
"pickup": "1000000061",
"distributionCenter": "CLSCL1",
"packages": [
{
"consigneeAddress": {
"address1": "apt 123",
"address2": "apt 123",
"address3": "SANTIAGO",
"city": "REGION METROPOLITANA",
"country": "CL",

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"email": "test@email.com",
"name": "Test Name",
"phone": "5555555",
"state": "REGION METROPOLITANA"
},
"packageDetails": {
"currency": "CLP",
"orderedProduct": "501180",
"packageDesc": "Desc",
"packageId": "TEST1234L1",
"weight": 10,
"weightUom": "G"
}
}
]
}
]
}
Match "shipments": [ from the payload