Complete Guide For Regular Expression

 

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R

egular expressions or regex puts a lot of people off, just because of its look at first glance. But once you master this it will open a whole new different level of doing string manipulation and the best part of it is that it can be used with mostly all of the programming language as well as with Linux commands. It can be used to find any kind of pattern that you can think of within the text and once you find the text you can do pretty much whatever you want to do with that text. By this example, you can get an idea of how powerful and useful regex is.

What is Regex?

If you are reading this post then most probably you already know what a regex is, if you don’t know here is a quick and easy definition

Regex stands for Regular Expression and is essentially an easy way to define a pattern of characters. The most common use of regex is in pattern identification, text mining, or input validation.

Let’s Get Regex...

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As we have seen regex can be used to find a pattern in the given sentence lets just start this up by finding just character. We are going to have a look at regex with python as this is the programming language that I love to work with.

Python has a built-in package called re, which can be used to work with Regular Expressions.

Just to search simple word

import re

word=” There is something that we are looking for “ re.search(“are”, word)
print(x) #['are']

Like every programming language, there are some special characters in regex and so we need to escape them in order to serve them. Let’s see what happens when we directly used them without using Escape Sequence

import re
word=”www.creatorghost.com 
x = re.findall(“.”, word)
print(x) #[‘w’, ‘w’, ‘w’, ‘.’, ‘c’, ‘r’, ‘e’, ‘a’, ‘t’, ‘o’, ‘r’, ‘g’, ‘h’, ‘o’, ‘s’, ‘t’, ‘.’, ‘c’, ‘o’, ‘m’]

Now let’s see the output using Escape Sequence ( \ )

import re
word=”www.creatorghost.com 
x = re.findall(“\.”, word) #Here only . will be searched
print(x) #[‘.’, ‘.’]

Let's have a look at all the Metacharacters

Python Regular Expression Quick Guide
^        Matches the beginning of a line
$        Matches the end of the line
.        Matches any character
\s       Matches whitespace 
?        0 or more time
\S       Matches any non-whitespace character
\r       Carriage return character
*        Repeats a character zero or more times
*?       Repeats a character zero or more times 
         (non-greedy)
+        Repeats a character one or more times
+?       Repeats a character one or more times 
         (non-greedy)
[aeiou]  Matches a single character in the listed set
[^XYZ]   Matches a single character not in the listed set
[a-z0-9] The set of characters can include a range
(        Indicates where string extraction is to start
)        Indicates where string extraction is to end

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Here we have seen all Regular Expressions now let’s see how we can combine all of them to get a wonderful result. Let’s see them with a real-world example.

Case 1 — Remove All Url From Text

Sample text

text1
text2
http://url.com/bla1/blah1/
text3
text4
http://url.com/bla2/blah2/
text5
text6
http://url.com/bla3/blah3/

Now let’s remove all of the URL

import re
text = re.sub(r'^https?:\/\/.*[\r\n]*', '', text, flags=re.MULTILINE)

Output:

text1
text2
text3
text4
text5
text6

Let's see how it worked. First of all, we used ^https that says starting with https and ? says it can either come one or it can’t come and then we used Escape Sequence to escape all our // and then the .* says any character can come any number of time and then we use \r\n to find all characters till the new line. And that how we were able to select all the URL and then we just python inbuilt re.sub to replace all URL with “ “ or just empty space.

Case 2 — Remove All Number From the text

text 123
text 234
text 3
text4
text5
text6

Now let's remove all number

import re
text =  re.sub(r'[0-9]+', '', text,flags=re.MULTILINE)

Output:

text
text
text
text
text
text

Case 3 — To Remove All Special Character

text @& for you 123

We want to remove all special character like @ &

import re
text =  re.sub(r'[^a-z0-9\s]', '', text,flags=re.MULTILINE)

Output:

text for you 123

Do we need to remember all the regular expressions?

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The simple answer is no, you don’t have to remember all the regular expressions if you want to find any type of regular expression for most of the time you can google it and find it on StackOverflow or any similar website .So you might've thinking then why should we study this, to be simply put you the latest need to know what code are you coping from the internet as not always it may suits your need sometimes when you need to customize it the knowledge of regex will surely help.

I hope you like this post and you can also see my previous post to know why python is mostly used nowadays. Thanks for reading.