Regular expressions, or regex, is used to identify patterns in a text. In this tutorial we will focus on regex in python, but during your degree you may see it in multiple languages such as javascript and scala.

Syntax

Before starting, here’s a list of useful special characters in Python’s regex library to refer back to.

  • . - any character except newline.
  • \n - new line.
  • \s - white space.
  • \t - tab.
  • \d - digit.
  • ^ - the start of a string or line.
  • $ - the end of a string or line.
  • * - repeats the previous character 0 or more times.
  • *? - repeats the previous character 0 or more times (non-greedily).
  • + - repeats the previous character 1 or more times.
  • +? - repeats the previous character 1 or more times (non-greedily).
  • [abc] - will match one of the characters in the set.
  • [^abc] - will match any character not in the set.
  • [a-z] - will match any character between a and z (inclusive).
  • a|b - matches either a or b.
  • {n} - matches exactly n occurrences.
  • {m,n} - matches as many as possible of m to n occurrences.

Getting started

To use regex in python you’ll need to use the re library. There’s also a regex library but it’s not yet needed. To find all the occurrences of a pattern you can use re.findall(), but to also get the location of those matches, re.finditer() is useful.

1
2
3
4
5
6
7
8
9

import re

str = "Using regex for the first time"

match = re.findall('e', str)
print(match)

for m in re.finditer('i.', str):
    print(m) # Match object, position, match

Running the above code in python returns

1
2
3
4

['e', 'e', 'e', 'e']
<re.Match object; span=(2, 4), match='in'>
<re.Match object; span=(21, 23), match='ir'>
<re.Match object; span=(27, 29), match='im'>

Match a collection

The [...] sign will match any character inside it while [^...] will match any character not inside it.

1
2
3
4
5
6
7
8
9
10
11
12

import re

str = "H0w 0ur m1nds c4n d0 4m4z1ng 7h1ng5!"
match = re.findall('[mwn]', str)
print(match)
# if we want to match lower case letters instead
match = re.findall('[a-z]', str)
print(match)
# The above expression won't match the 'H', space and '!'
# To get them, match everything except the numbers
match = re.findall('[^0-9]', str)
print(match)

Running the above code in python returns

1
2
3
4

['w', 'm', 'n', 'n', 'm', 'n', 'n']
['w', 'u', 'r', 'm', 'n', 'd', 's', 'c', 'n', 'd', 'm', 'z', 'n', 'g', 'h', 'n', 'g']
['H', 'w', ' ', 'u', 'r', ' ', 'm', 'n', 'd', 's', ' ', 'c', 'n', ' ', 'd', ' ', 'm',
 'z', 'n', 'g', ' ', 'h', 'n', 'g', '!']

Match repeating characters

The + character can be used to match 1 or more occurrence while * matches 0 or more occurrences. They are both greedy unless directly followed by a ?.

1
2
3
4
5
6
7
8
9
10
11
12
13
14

import re

str = "The given student ids are 46125656, 87654321, 46464321, 46561256"
match = re.findall('[0-9]+', str)
print(match)
# Ending in 56
match = re.findall('[0-9]+56', str)
print(match)
# Ending in 56 (non-greedy)
match = re.findall('[0-9]+?56', str)
print(match)
# Matches 0 or more occurrences of the numbers before 56
match = re.findall('[0-9]*?56', str)
print(match)

Running the above code in python returns

1
2
3
4

['46125656', '87654321', '46464321', '46561256']
['46125656', '46561256']
['461256', '4656', '1256']
['461256', '56', '4656', '1256']

Exercises

This link can be helpful to debug your code.

  1. Find all the occurrences of b. Example sentence: he made a bad bid for the bed.
  2. Find all the occurrences of the word ever. Example sentence: We are never, ever, ever, ever getting back together. Make sure not to include everyone.
  3. Find all the occurrences of any of the characters in the word ever. Example sentence: We are never, ever, ever, ever getting back together. Make sure not to include everyone.
  4. Find all the occurrences of the phrase ev or re. Example sentence: We are never, ever, ever, ever getting back together. Make sure not to include everyone.
  5. Find all occurrences of substrings starting with b and ending with d. Example sentence: Abid made a baaad bid for the bed and got hit in the abdomen.
  6. Advanced: Find all words in a text. The text can contain multiple sentences ending in one of !?.. Only words containing letters from the English alphabet should be included.