Regular Expression Basics in Python

In this post, we will be discussing the following contents...

• Why we use RE
• Finding text without RE
• Finding text with RE
• Creating Regex Object
• Matching Regex Object
• Review

Why use Regular Expression???

We usually search text by pressing Ctrl + F and typing the word you want. 

Regular Expression goes more further and allows you to specify a pattern of text to search for. Let us take an example of a Telephone number from the US or Canada. There they have a pattern as 3 number then hyphen, 4 number then a hyphen then 4 number. 

So the correct way is::: 415-555-1234

But the incorrect way is ::: 4,155,551,234.

What if we need to find all the numbers of this pattern in the file, not just a single number then here we will be using Regular Expression...

But is there a way, in which we are not using a regular expression?

Finding Text without Regular Expression...

Yes, there is.

So, You know the pattern three numbers, a hyphen, three numbers a hyphen, and four numbers. 

Ex.: 415-555-4242

Try it yourself first

Isn't it very lengthy...

And Hence, we have Regular Expression which is going to convert these 17-18 lines of code to 3-4 lines of code...

Finding Pattern with Regular Expression...

Regular Expression called regex for short, are descriptions for patterns of texts. For example, \d in regex stands for digit character -- i.e., a single number from 0 to 9.

So the regex \d\d\d-\d\d\d-\d\d\d\d is used by Pyhton to match the same text the previous isPhoneNumber() function did.

Regular Expression is more sophisticated. Eg. Adding 3 in curly bracket ({3})after the pattern is like saying. "Match this pattern three times."

        So, we have : \d{3}-\d{3}-\d{4}

Yaa isn't it very simple...

    Creating Regex Objects...

        All regex functions are in the re module.

                >>> import re

        

        Passing a string value representing your regular expression to re.compile() returns a Regex pattern object (or simply, a Regex object).

                >>> phoneNumRegex = re.compile(r'\d\d\d-\d\d\d-\d\d\d\d')

        Here, you see r' '. This makes the string inside the quotes a Raw String. If you forget to write this you need to write it like, \\d\\d\\d-\\d\\d\\d-\\d\\d\\d\\d.

    Matching Regex Object...

            search() method -- searches the string it is passed for any matches to the regex. Returns None if the regex is not formed.

        

            group() method -- returns the actual matched text from the searched string.

Enter this in your interactive shell...

Let's review...

While there are several steps in regular expression in Python, each step is fairly simple.

• Import the regex module with import re.
• Create a Regex Object with the re.compile() function. {Remember to use raw string}
• Pass the string you want to search into the Regex object's search() method. This returns a Matched object.
• Call the Matched object's group() method to return a string of actual matched text.

Here is the video Explanation of the Basics of Regular Expression, go check this out...

December 15, 2020 Tags : Python , Regular Expression

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