Month: April 2021

Hello everybody,

It’s Michael, and today’s post will cover regular expressions in Python. I know I already did a Java lesson on RegEx (the colloquial term for regular expressions-here’s the link to that lesson: Java Lesson 18: RegEx (or Regular Expressions)) but I wanted to cover how to use regular expressions in Python, so here goes.

To start working with regular expressions in Python, import the regular expressions module using this line of code-import re.

Now, here’s a simple example of regular expressions in Python:

text = "Jupyter Notebook is awesome!!!"

print(re.findall('e', text))

['e', 'e', 'e', 'e']

text2 = "Tonight is a very beautiful spring night"

print(re.search("ht$",text2))

<re.Match object; span=(38, 40), match='ht'>

In this example, I am using the search function to find the pattern ht$ anywhere in the string. An important thing to note is that, unlike the findall function, the search function only looks for one pattern/string/character match in the string being analyzed (in this case, text2).

• Similar to the findall function, you’ll need to include two parameters for the search function-the pattern/string/character you’re searching for and the string where you are looking for the pattern/string/character.

Now, you’re probably wondering what ht$ means. The dollar sign ($) is called a metacharacter and in the context of regular expressions, metacharacters help define more specific search criteria for a pattern/string/character you are looking for. In the case of ht$, the search function is looking for any part of the text2 string that ends with ht; a Match object is returned if the search function finds any part of the String that ends with ht. In this case, a Match object was returned; the span attribute of the Match object will tell you where in the string a match was found (span shows you index positions in the String where the match was found). For text2, the match starts at index 38 and ends at index 39 (index 40 won’t be considered part of the match)-in other words, the match is found between the 39th and 40th positions in text2 (recall that string indexing starts with 0).

Here’s a list of other useful regex metacharacters:

• []-find a set of characters
  • example: [a, e, i, o, u, y] can be used to find all of the vowels in text2 (and yes I’ll consider y as a vowel)
• \-use a special sequence (more on this later)
• .-find any character (except the \n newline character)
  • example: n...t can be used in text2 to find any word in the string that starts with n, ends with t, and has any three letters in between
• ^-find any part of the string that starts with a certain character/pattern
  • example: ^To can be used to find any part of text2 that starts with “To”
• $-find any part of the string that ends with a certain character/pattern (I explained this metacharacter in the above example)
• *-find none (or more) occurrences of a certain character/pattern in a string
  • example: ni* can be used to find any amount (or no amount) of occurrences of the pattern “ni” in text2.
• +-find at least one occurrence of a certain character/pattern in a string
  • example: ni+ can be used to find at least one occurrence of the pattern “ni” in text2
• {}-find a specific number of occurrences of a certain character/pattern in a string
  • example: ni{1} can be used to find one AND ONLY ONE occurrence of the pattern “ni” in the string text2.
• |-find a match that contains either pattern/string
  • example: tonight|today can be used to find out whether text2 contains either tonight or today.
  • note: this is the same operator that you’d use as an OR statement in conditional logic but it takes a slightly different albeit conceptually similar meaning when dealing with Python regex.

Now I know I mentioned special sequences in the list above, so let’s see some special sequences in action:

text3 = "Today's date is 04/10/2021"

print(re.split("\d{2}/\d{2}/\d{4}", text3))

["Today's date is ", '']

Here is a list of all the special sequences that Python regex uses:

• \A-looks for a match if certain character(s) are at the beginning of the string
  • example: \ATo can be used to see if text3 starts with the characters “To”
• \b-looks for a match if certain character(s) are at the beginning or end of a word
  • example: \bda can be used to see if there are any words in text3 that start with the characters “da”. However, if you want to see if “da” can be found at the end of a word, use the syntax da\b
• \B-looks for a match if certain character(s) are present BUT NOT at the beginning or end of a word
  • example: \Bda can be used to see if there are any words in text3 that contain but don’t begin with the characters “da”. Likewise, da\B can be used to see if there are any words in text3 that contain but don’t end with the characters “da”.
• \d-looks for digits in the string (I discussed this sequence in the example above)
• \D-looks for non-digits in the string
  • example: \D can used to return all of the non-digit characters in text3.
  • Yes, whitespace counts as a character too.
• \s-looks for all of the whitespace characters in the string
  • example: \s can be used to return a list of all the whitespace characters in text3, of which there are three.
• \S-looks for all of the non-whitespace characters in the string
  • example: \S can be used to return a list of all the non-whitespace characters in text3
• \w-looks for all of the word characters in the string; in case you’re wondering, the word characters are the letters of the alphabet, the digits 0-9, and the underscore (_)
  • example: \w can be used to return a list of all the word characters in text3
• \W-looks for all of the non-word characters in the string (in other words, anything that’s not a letter, digit, or underscore)
  • example: \W can be used to return a list of all the non-word characters in text3
• \Z-looks for a match if certain character(s) are at the end of the string
  • example: 21\Z can be used to see if text3 ends with the characters “21”.

Now I’ve shown you how to split a string with regex, find all instances of a certain character pattern in a string, and retrieve a match object using regex. However, what if you wanted to replace one character pattern with another? Here’s an example of this:

text4 = "Today was a beautiful Monday afternoon!"

print(re.sub("Mon", "Tues", text4))

Today was a beautiful Tuesday afternoon!

Now, before I go, I want to discuss one more Python regex concept-sets. In regex, sets are sets of characters inside square brackets with a special meaning.

Let’s check out an example of sets below:

text5 = "His address is 742 Evergreen Terrace. His phone number is 413-234-9080. His date of birth is 09/12/1971"

print(re.findall("[0-9][0-9]", text5))

['74', '41', '23', '90', '80', '09', '12', '19', '71']

In this example, I am using the set [0-9][0-9] to find all two-digit sequences in text5.

You’re probably wondering what [0-9][0-9] does in the context of regex. The set [0-9][0-9] looks for all two-digit sequences in text5 between 00 and 99.

One interesting things about sets is that, between them, special sequences, and metacharacters, they are the most customizable of the three regex elements (though you could argue that metacharacters are widely customizable as well). In the example above, I could’ve used the set [0-9][0-9][0-9] to look for all three-digit sequences in text5 between 000 and 999. But what if I didn’t want to use the 00-99 digit sequence range? Let’s say I wanted to look for all two-digit sequences in text5 between 00 and 49; all I need to do is specify the set [0-4][0-9] in the first parameter of the findall function.

What other regex sets can you use with Python? Here’s a list of them: