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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. > Regular expression is a group of characters 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.
# 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]() 1. [Basic Matchers]()
2. [Meta character]() 2. [Meta character]()
@ -19,17 +29,16 @@ from a string based upon a pattern match, and so much more.
## 1. Basic Matchers ## 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
A regular expression is just a pattern of letters and digits that we used to search in a text. For example the `cat` means: the letter `c`, followed by the letter `a`, followed by the letter `t`.
regular expression `cat` means: the letter `c`, followed by the letter `a`, followed by the letter `t`.
<pre> <pre>
"cat" => The <strong><u>cat</u></strong> sat on the mat "cat" => The <strong><u>cat</u></strong> sat on the mat
</pre> </pre>
The regular expression `123` matches the string "123". The regular expression is matched against an input string The regular expression `123` matches the string "123". The regular expression is matched against an input string by comparing each
by comparing each character in the regular expression to each character in the input string, one after another. character in the regular expression to each character in the input string, one after another. Regular expressions are normally
Regular expressions are normally case-sensitive so the regular expression `Cat` would not match the string "cat". case-sensitive so the regular expression `Cat` would not match the string "cat".
<pre> <pre>
"Cat" => The cat sat on the <strong><u>Cat</u></strong> "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 ## 2. Meta Characters
Meta characters are the building blocks of the regular expressions. Some meta characters have a special meaning Meta characters are the building blocks of the regular expressions. Meta characters do not stand for themselves but instead are
that are written inside the square brackets. The meta character are as follows: 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| |Meta character|Description|
|:----:|----| |:----:|----|
@ -57,9 +67,9 @@ that are written inside the square brackets. The meta character are as follows:
## 2.1 Full stop ## 2.1 Full stop
Full stop `.` is the simplest example of meta character. The meta character `.` matches any single character. It Full stop `.` is the simplest example of meta character. The meta character `.` matches any single character. It will not match return
will not match return or new line characters. For example the regular expression `.ar` means: any character, or new line characters. For example the regular expression `.ar` means: any character, followed by the letter `a`, followed by the
followed by the letter `a`, followed by the letter `r`. letter `r`.
<pre> <pre>
".ar" => The <strong><u>car</u></strong> <strong><u>par</u></strong>ked in the <strong><u>gar</u></strong>age. ".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 ## 2.2 Character set
Character sets are also called character class. Square brackets are used to specify character sets. Use hyphen Character sets are also called character class. Square brackets are used to specify character sets. Use hyphen inside character set to
inside character set to specify the characters range. The order of the character range inside square brackets specify the characters range. The order of the character range inside square brackets doesn't matter. For example the regular
doesn't matter. For example the regular expression `[Tt]he` means: an uppercase `T` or lowercase `t`, followed by expression `[Tt]he` means: an uppercase `T` or lowercase `t`, followed by the letter `h`, followed by the letter `e`.
the letter `h`, followed by the letter `e`.
<pre> <pre>
"[Tt]he" => <strong><u>The</u></strong> car parked in <strong><u>the</u></strong> garage. "[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 ### 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 In general the caret symbol represents the start of the string, but when it is typed after the opening square bracket it negates the
bracket it negates the character set. For example the regular expression `[^c]ar` means: any character except character set. For example the regular expression `[^c]ar` means: any character except `c`, followed by the character `a`, followed by
`c`, followed by the character `a`, followed by the letter `r`. the letter `r`.
<pre> <pre>
"[^c]ar" => The car <strong><u>par</u></strong>ked in the <strong><u>gar</u></strong>age. "[^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 ### 2.2.2 Repeating character set
We can repeat a character class by using `+`, `*` or `?` operators. For example the regular expression `[a-z]+` We can repeat a character class by using `+`, `*` or `?` operators. For example the regular expression `[a-z]+` means: any number of
means: any number of lowercase letters in a row. lowercase letters in a row.
<pre> <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>. "[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>.