Regexes: first match

import re

text = 'The black cat climed'
match = re.search(r'lac', text)
if match:
    print("Matching")       # Matching
    print(match.group(0))   # lac

match = re.search(r'dog', text)
if match:
    print("Matching")
else:
    print("Did NOT match")
    print(match)     # None

In this first example we are going to see the basic mechanism of using Regular Expressions in Python. Therefore we are using a very simple regular expression. We have a string in the text variable and we would like to know if the series of characters lac can be found in it.

We need to import the re library that implements the regex engine. It has several methods. In this example we are using the search method. t receives two parameters. The first one is the regular expression and the second one is the string in which we are searching. You might have noticed that the regex string is prefixed with the letter r. That tells Python that this is a raw string. In this case it is not needed as the regex does not contain and backslashes \ but I feel it is a good idea to always use it in regular expression so we won't forget it.

The search method returns either an object representing the match or None, if it could not find any match.

We can use this value in a conditional statement that will be True if there was a match and it will be False if there was not match and the value in None.

If there is a match you can call the group() method. Passing 0 to it will return the actual substring that was matched. In this specific example this is not too interesting. We know we were looking for lac so if there is a match we know that it has matched lac. It will be much more interesting in the next example when we make the regex more flexible.

In this first example we use a very simple regular expression that did not have any special characters. We knew exactly what we are looking for. In reality in such cases there is no need to use the re library. We could simply use the text.find('lac') or we could use the in operator and ask: 'lac' in text, but I wanted to show the syntax used with the re library.

In the second example we were searching for dog, but as there is no dog in this text the search() method will return None.

• r
• re
• search
• group