Python regex to find sequences of one upper case letter followed by lower case letters

When working with text processing, you often need to find sequences of one uppercase letter followed by lowercase letters. Python's regex module provides an efficient way to match such patterns using regular expressions.

Understanding the Pattern

The regex pattern [A-Z][a-z]+ matches one uppercase letter followed by one or more lowercase letters. Let's break it down ?

• [A-Z] − matches exactly one uppercase letter
• [a-z]+ − matches one or more lowercase letters
• $ − ensures the pattern matches to the end of string

Basic Pattern Matching

Here's a simple function to check if a string follows the pattern ?

import re

def match_string(my_string):
    pattern = '[A-Z][a-z]+$'
    
    if re.search(pattern, my_string):
        return 'The string meets the required condition'
    else:
        return 'The string does not meet the required condition'

# Test with different strings
test_strings = ["Python", "python", "PYTHON", "PyThOn", "P"]

for string in test_strings:
    print(f"String: {string}")
    print(match_string(string))
    print()

String: Python
The string meets the required condition

String: python
The string does not meet the required condition

String: PYTHON
The string does not meet the required condition

String: PyThOn
The string does not meet the required condition

String: P
The string does not meet the required condition

Finding All Matches in Text

To find all sequences within a longer text, use findall() method ?

import re

text = "Hello World! Python Programming Language Java Script"
pattern = '[A-Z][a-z]+'

matches = re.findall(pattern, text)
print("Found sequences:", matches)

# Find matches with their positions
for match in re.finditer(pattern, text):
    print(f"Match: '{match.group()}' at position {match.start()}-{match.end()}")

Found sequences: ['Hello', 'World', 'Python', 'Programming', 'Language', 'Java', 'Script']
Match: 'Hello' at position 0-5
Match: 'World' at position 6-11
Match: 'Python' at position 13-19
Match: 'Programming' at position 20-31
Match: 'Language' at position 32-40
Match: 'Java' at position 41-45
Match: 'Script' at position 46-52

Comparison of Regex Methods

Practical Example

Extract proper nouns (capitalized words) from a sentence ?

import re

sentence = "Alice and Bob visited New York and Paris last Summer."
pattern = '[A-Z][a-z]+'

proper_nouns = re.findall(pattern, sentence)
print("Proper nouns found:", proper_nouns)

# Count occurrences
print(f"Total proper nouns: {len(proper_nouns)}")

Proper nouns found: ['Alice', 'Bob', 'New', 'York', 'Paris', 'Summer']
Total proper nouns: 6

Conclusion

Use the regex pattern [A-Z][a-z]+ to find sequences of one uppercase letter followed by lowercase letters. The search() method checks for existence, while findall() extracts all matches from text.