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# What is Regular Expression?
## What is Regular Expression?
Regular expression is a group of character or symbols which is used to find a specific pattern from a text.
The word "Regular expression" is a mouthful, you will usually find the term abbreviated as "regex" or
"regexp". Regular expression is used for replacing a text withing a string, validating form, extract a substring
from a string based upon a pattern match, and so much more.
> Regular expression is a group of characters or symbols which is used to find a specific pattern from a text.
# Table of Contents
A regular expression is a pattern that is matched against a subject string from left to right. The word "Regular expression" is a
mouthful, you will usually find the term abbreviated as "regex" or "regexp". Regular expression is used for replacing a text withing
a string, validating form, extract a substring from a string based upon a pattern match, and so much more.
Imagine you are writing an application and you want to set the rules when user chosing their username. We want the username can
contains letter, number, underscore and hyphen. We also want to limit the number of characters in username so it does not look ugly.
We use the following regular expression to validate a username:
<center>![Example](http://i.imgur.com/UrDb9qc.png)</center>
Above regular expression can accepts the strings "john_doe", "jo-hn\_doe" and "john12\_as". It does not match "Jo" because that string
contains uppercase letter and also it is too short.
## Table of Contents
1. [Basic Matchers]()
2. [Meta character]()
@ -19,17 +29,16 @@ from a string based upon a pattern match, and so much more.
## 1. Basic Matchers
A regular expression is just a pattern of letters and digits that we used to search in a text. For example the
regular expression `cat` means: the letter `c`, followed by the letter `a`, followed by the letter `t`.
A regular expression is just a pattern of letters and digits that we used to search in a text. For example the regular expression
`cat` means: the letter `c`, followed by the letter `a`, followed by the letter `t`.
<pre>
"cat" => The <strong><u>cat</u></strong> sat on the mat
</pre>
The regular expression `123` matches the string "123". The regular expression is matched against an input string
by comparing each character in the regular expression to each character in the input string, one after another.
Regular expressions are normally case-sensitive so the regular expression `Cat` would not match the string "cat".
The regular expression `123` matches the string "123". The regular expression is matched against an input string by comparing each
character in the regular expression to each character in the input string, one after another. Regular expressions are normally
case-sensitive so the regular expression `Cat` would not match the string "cat".
<pre>
"Cat" => The cat sat on the <strong><u>Cat</u></strong>
@ -37,8 +46,9 @@ Regular expressions are normally case-sensitive so the regular expression `Cat`
## 2. Meta Characters
Meta characters are the building blocks of the regular expressions. Some meta characters have a special meaning
that are written inside the square brackets. The meta character are as follows:
Meta characters are the building blocks of the regular expressions. Meta characters do not stand for themselves but instead are
interpreted in some special way. Some meta characters have a special meaning that are written inside the square brackets.
The meta character are as follows:
|Meta character|Description|
|:----:|----|
@ -57,9 +67,9 @@ that are written inside the square brackets. The meta character are as follows:
## 2.1 Full stop
Full stop `.` is the simplest example of meta character. The meta character `.` matches any single character. It
will not match return or new line characters. For example the regular expression `.ar` means: any character,
followed by the letter `a`, followed by the letter `r`.
Full stop `.` is the simplest example of meta character. The meta character `.` matches any single character. It will not match return
or new line characters. For example the regular expression `.ar` means: any character, followed by the letter `a`, followed by the
letter `r`.
<pre>
".ar" => The <strong><u>car</u></strong> <strong><u>par</u></strong>ked in the <strong><u>gar</u></strong>age.
@ -67,10 +77,9 @@ followed by the letter `a`, followed by the letter `r`.
## 2.2 Character set
Character sets are also called character class. Square brackets are used to specify character sets. Use hyphen
inside character set to specify the characters range. The order of the character range inside square brackets
doesn't matter. For example the regular expression `[Tt]he` means: an uppercase `T` or lowercase `t`, followed by
the letter `h`, followed by the letter `e`.
Character sets are also called character class. Square brackets are used to specify character sets. Use hyphen inside character set to
specify the characters range. The order of the character range inside square brackets doesn't matter. For example the regular
expression `[Tt]he` means: an uppercase `T` or lowercase `t`, followed by the letter `h`, followed by the letter `e`.
<pre>
"[Tt]he" => <strong><u>The</u></strong> car parked in <strong><u>the</u></strong> garage.
@ -78,9 +87,9 @@ the letter `h`, followed by the letter `e`.
### 2.2.1 Negated character set
In general the caret symbol represents the start of the string, but when it is typed after the opening square
bracket it negates the character set. For example the regular expression `[^c]ar` means: any character except
`c`, followed by the character `a`, followed by the letter `r`.
In general the caret symbol represents the start of the string, but when it is typed after the opening square bracket it negates the
character set. For example the regular expression `[^c]ar` means: any character except `c`, followed by the character `a`, followed by
the letter `r`.
<pre>
"[^c]ar" => The car <strong><u>par</u></strong>ked in the <strong><u>gar</u></strong>age.
@ -89,8 +98,8 @@ bracket it negates the character set. For example the regular expression `[^c]ar
### 2.2.2 Repeating character set
We can repeat a character class by using `+`, `*` or `?` operators. For example the regular expression `[a-z]+`
means: any number of lowercase letters in a row.
We can repeat a character class by using `+`, `*` or `?` operators. For example the regular expression `[a-z]+` means: any number of
lowercase letters in a row.
<pre>
"[a-z]+" => <strong><u>The</u></strong> <strong><u>car</u></strong> <strong><u>parked</u></strong> <strong><u>in</u></strong> <strong><u>the</u></strong> <strong><u>garage</u></strong>.