IDENTIFIER

Let’s get started with some basic knowledge first. So-called identifiers define what kind of character should be at a certain place. Here you have some examples:

| |REGEX IDENTIFIERS |                                      |
|--------------------|--------------------------------------|
| IDENTIFIER         | DESCRIPTION                          |
| \d                 | Some digit                           |
| \D                 | Everything BUT a digit               |
| \s                 | White space                          |
| \S                 | Everything BUT a white space         |
| \w                 | Some letter                          |
| \W                 | Everything BUT a letter              |
| .                  | Every character except for new lines |
| \b                 | White spaces around a word           |
| \.                 | A dot                                |

MODIFIER

The modifiers extend the regular expressions and the identifiers. They might be seen as some kind of operator for regular expressions.

| REGEX MODIFIERS |                                            |
|-----------------|--------------------------------------------|
| MODIFIER        | DESCRIPTION                                |
| {x,y}           | A number that has a length between x and y |
| +               | At least one                               |
| ?               | None or one                                |
| *               | Everything                                 |
| $               | At the end of a string                     |
| ^               | At the beginning of a string               |
| |               | Either Or                                  |
|                 | Example: x | y = either x or y             |
| []              | Value range                                |
| {x}             | x times                                    |
| {x,y}           | x to y times                               |

ESCAPE CHARACTERS

| REGEX ESCAPE CHARATCERS |             |
|-------------------------|-------------|
| CHARACTER               | DESCRIPTION |
| \n                      | New Line    |
| \t                      | Tab         |
| \s                      | White Space |

APPLYING REGULAR EXPRESSIONS

FINDING STRINGS

In order to apply these regular expressions in Python, we need to import the module re .

import re

Now we can start by trying to find some patterns in our strings.

text = '''
Mike is 20 years old and George is 29!
My grandma is even 104 years old!
'''
ages = re.findall( r'\d{1,3}' , text)
print (ages)

## ['20', '29', '104']

In this example, we have a text with three ages in it. What we want to do is to filter these out and print them separately.

As you can see, we use the function findall in order to apply the regular expression onto our string. In this case, we are looking for numbers that are one to three digits long. Notice that we are using an r character before we write our expression. This indicates that the given string is a regular expression.

At the end, we print our result and get the following output:

[‘20’, ‘29’, ‘104’]

MATCHING STRINGS

What we can also do is to check if a string matches a certain regular expression. For example, we can apply our regular expression for mails here.

import re

text = 'test@mail.com'

result = re.fullmatch( r'^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$' , text)

if result != None :
  print ( 'VALID!' )
else :
  print ( 'INVALID!' )

We are not going to talk about the regular expression itself here. It is very long and complicated. But what we see here is a new function called fullmatch . This function returns the checked string if it matches the regular expression. In this case, this happens when the string has a valid mail format.

If the expression doesn’t match the string, the function returns None . In our example above, we get the message “VALID!” since the expression is met. If we enter something like “Hello World!”, we will get the other message.

import re
text = '''
Mike is 20 years old and George is 29!
My grandma is even 104 years old!
'''
text = re.sub( r'\d{1,3}' , '100' , text)
print (text)

## 
## Mike is 100 years old and George is 100!
## My grandma is even 100 years old!