Python Match.start Method

last modified April 20, 2025

Introduction to Match.start

The Match.start method is part of Python's re module. It returns the starting position of the match within the searched string.

This method is called on a match object returned by functions like search or match. It provides valuable position information about where matches occur.

Understanding match positions is crucial for text processing tasks like extraction, validation, and manipulation. The start method helps precisely locate matches in the input string.

Basic Syntax

The syntax for Match.start is straightforward:

match.start([group])

The optional group parameter specifies which capturing group's start position to return. Defaults to 0 (the entire match).

Basic Match.start Usage

Let's start with a simple example of finding a word's position in text.

basic_start.py

#!/usr/bin/python

import re

text = "The quick brown fox jumps over the lazy dog"
pattern = re.compile(r'fox')

match = pattern.search(text)
if match:
    print(f"Found 'fox' starting at position {match.start()}")

This example shows how to get the starting index of a simple match. The position is zero-based, counting from the start of the string.

match = pattern.search(text)

We search for the pattern in the text, which returns a match object if found. This object contains match position information.

match.start()

Calling start without arguments returns the starting index of the entire match. Here it would return 16.

Using Match.start with Groups

We can get start positions for specific capturing groups in the pattern.

group_start.py

#!/usr/bin/python

import re

text = "Date: 2023-12-25"
pattern = re.compile(r'Date: (\d{4})-(\d{2})-(\d{2})')

match = pattern.search(text)
if match:
    print(f"Full match starts at {match.start()}")
    print(f"Year starts at {match.start(1)}")
    print(f"Month starts at {match.start(2)}")
    print(f"Day starts at {match.start(3)}")

This demonstrates getting start positions for different capturing groups. Group 0 is the full match, while groups 1-3 are the captured components.

Handling Multiple Matches

When processing multiple matches, start helps track each.

multiple_matches.py

#!/usr/bin/python

import re

text = "cat bat hat mat"
pattern = re.compile(r'\w+at')

for match in pattern.finditer(text):
    word = match.group()
    print(f"Found '{word}' at position {match.start()}")

This example processes all matches in the text, printing each word and its starting position. The finditer method yields match objects.

Match.start with Named Groups

Named groups provide more readable access to match components.

named_groups.py

#!/usr/bin/python

import re

text = "Temperature: 23.5°C"
pattern = re.compile(r'Temperature: (?P<value>\d+\.\d+)°(?P<unit>[CF])')

match = pattern.search(text)
if match:
    print(f"Temperature starts at {match.start('value')}")
    print(f"Unit starts at {match.start('unit')}")

Named groups make patterns more maintainable. We can reference groups by name when getting their start positions.

Error Handling with Match.start

It's important to handle cases where groups don't participate in matches.

error_handling.py

#!/usr/bin/python

import re

text = "color: red"
pattern = re.compile(r'color: (red|blue)(?:, shade: (\w+))?')

match = pattern.search(text)
if match:
    try:
        print(f"Shade starts at {match.start(2)}")
    except IndexError:
        print("Shade group didn't participate in match")

Optional groups might not participate in matches. Accessing their start positions raises IndexError, which we handle gracefully.

Advanced: Match.start in Replacement Functions

start is useful in replacement functions for complex substitutions.

replacement_function.py

#!/usr/bin/python

import re

text = "10 apples, 5 oranges, 3 bananas"
pattern = re.compile(r'(\d+) (\w+)')

def double_odd(match):
    count = int(match.group(1))
    if count % 2 == 1:
        start = match.start(1)
        end = match.end(1)
        return f"{count*2}{match.string[start+len(str(count)):end]} {match.group(2)}"
    return match.group()

result = pattern.sub(double_odd, text)
print(result)

This example doubles odd numbers while preserving formatting. We use start and end to precisely modify the match.

Performance Considerations

The start method is highly optimized and has minimal performance impact. However, unnecessary position calculations in tight loops should be avoided.

For most use cases, the overhead of calling start is negligible compared to the actual matching operation.

Best Practices

When using Match.start, follow these best practices:

Always check if a match was found before calling start
Handle potential IndexError for optional groups
Use named groups for better code readability
Combine with end for complete match position information
Document expected group indices when using numbered groups

Source

Python Match.start() documentation

This tutorial covered the essential aspects of Python's Match.start method. Mastering match positions will enhance your text processing capabilities.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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