Python · Some intermediate topics
Regular Expressions
The full text of this chapter, to read or revisit. To practise, do the exercises.
▶ Do the exercisesExercise 1 — What are Regular Expressions?
A regular expression (or regex) is a special pattern used to search, match, and manipulate text. Think of it like a super-powered `in` operator. Instead of searching for an exact word, you can describe what the text looks like.
For example: "find all words that start with a capital letter" or "check if a string is a valid email". Python uses the `re` module for regex.
Exercise 2 — re.search() — Find a Match
`re.search(pattern, text)` scans the text and returns the first match, or `None` if nothing matches. Here, `\d+` means "one or more digits" and `.group()` returns the matched text.
import re
text = "Pyrochu is level 25"
match = re.search(r"level \d+", text)
if match:
print(f"Found: {match.group()}")
else:
print("No match")
Always check if the result is `None` before calling `.group()`, or you'll get an error!
Raw Strings
The `r` before the string (`r"\d+"`) means raw string — it tells Python not to interpret backslashes. Always use raw strings for regex patterns!
Exercise 3 — Basic Patterns
Here are the most common regex patterns:
Common Regex Patterns
`\d` — any digit (0-9)
`\w` — any word character (letter, digit, underscore)
`\s` — any whitespace (space, tab, newline)
`.` — any character except newline
`+` — one or more of the previous
`*` — zero or more of the previous
`?` — zero or one of the previous
Let's see `re.findall()` in action — it returns all matches as a list, not just the first one:
import re
text = "Aquachu HP: 100, Level: 25"
digits = re.findall(r"\d+", text)
words = re.findall(r"\w+", text)
print("Digits:", digits)
print("Words:", words)
Exercise 4 — findall Result
import re
text = "Aquachu: 100 HP, Pyrochu: 85 HP"
result = re.findall(r"\d+", text)
print(result)
Options
• ```["100", "85"]```
• ```[100, 85]```
• ```["100 HP", "85 HP"]```
• ```["Aquachu", "Pyrochu"]```
Hint : `\d+` matches digits only. `findall()` returns matches as ***strings***, not numbers.
Exercise 5 — Find All Digits
Complete the code to find all numbers in the text.
import re
text = "Florachu has 80 HP and 15 attack"
numbers = re.____(r"\d+", text)
print(numbers)
Options: findall search match split replace
Hint : Which `re` function returns ALL matches as a list?
Exercise 6 — re.sub() — Replace Matches
`re.sub(pattern, replacement, text)` replaces all matches with a new string. It's like `str.replace()` but with the power of regex!
import re
text = "Aquachu has 100 HP"
result = re.sub(r"\d+", "???", text)
print(result)
Every group of digits got replaced by `"???"`. This is great for censoring, formatting, or cleaning data.
Exercise 7 — Replace with Regex
Complete the code to replace all numbers with `"X"`.
import re
text = "Level 25, HP 100"
result = re.____(r"\d+", ____, text)
print(result)
Options: sub "X" findall "\\d+" search
Hint : Which `re` function replaces matches? The second argument is the replacement string.
Exercise 8 — Character Classes and Groups
You can define your own character sets with square brackets `[]`. And use parentheses `()` to capture groups.
import re
text = "Aquachu-Water, Pyrochu-Fire"
pairs = re.findall(r"(\w+)-(\w+)", text)
print(pairs)
for name, element in pairs:
print(f"{name} is {element} type")
`(\w+)-(\w+)` captures two groups separated by a dash. `findall()` returns a list of tuples when you use groups!
Character Classes
`[abc]` matches a, b, or c
`[A-Z]` matches any uppercase letter
`[0-9]` is the same as `\d`
`()` captures groups you can extract separately
Exercise 9 — Groups
import re
text = "Aquachu:Water"
match = re.search(r"(\w+):(\w+)", text)
print(match.group(2))
Options
• ```"Aquachu:Water"```
• ```"Aquachu"```
• ```"Water"```
• ```"(\w+):(\w+)"```
Hint : `.group(1)` is the first capture group, `.group(2)` is the second. Which word comes after the colon?
Exercise 10 — Anchors: ^ and $
Anchors don't match characters — they match positions. `^` means "start of string" and `$` means "end of string".
import re
names = ["Aquachu", "Florachu", "Zap"]
for name in names:
if re.search(r"^[AZ]", name):
print(f"{name} starts with A or Z")
else:
print(f"{name} does NOT start with A or Z")
if re.search(r"chu$", name):
print(f"{name} ends with 'chu'")
else:
print(f"{name} does NOT end with 'chu'")
Exercise 11 — Anchor Pattern
Complete the regex to check if the name starts with `"Aqua"`.
import re
name = "Aquachu"
if re.search(r"____Aqua", name):
print("Starts with Aqua!")
Options: ^ $ \\d \\w .
Hint : Which anchor means "start of string"?
Exercise 12 — Regex Patterns
Match each regex pattern with its meaning:
Items to match
• \d+
• \w+
• \s
• .
• ^
• $
Definitions
• One or more digits
• One or more word characters
• A whitespace character
• Any character (except newline)
• Start of string
• End of string
Exercise 13 — Codémon Data Parser
Parse a Codémon database string using regex!
Given: `"Aquachu(Water,Lv.15) Pyrochu(Fire,Lv.25) Florachu(Plant,Lv.10)"`
1. Use `re.findall()` with groups to extract: name, element, and level for each Codémon
2. Print each as: `"Aquachu - Water type, Level 15"`
3. Use `re.sub()` to replace all levels with `"Lv.99"`
4. Print the modified string
Starter code
import re
data = "Aquachu(Water,Lv.15) Pyrochu(Fire,Lv.25) Florachu(Plant,Lv.10)"
# 1 & 2: Extract and print each Codémon
# 3 & 4: Replace all levels with Lv.99