5 Best Ways to Find the Exact Positions of Each Match in Python’s Regular Expressions

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💡 Problem Formulation: When working with Python’s regular expressions, it is often necessary not only to find if a pattern exists but also to locate the exact indices where each occurrence of the pattern is found within the string. For instance, given the input string “cat, bat, rat, sat” and the pattern “at”, the desired output should be the start and end indices of each match: [(1, 3), (6, 8), (11, 13), (16, 18)].

Method 1: Using re.finditer()

With re.finditer(), you can iterate over all non-overlapping matches in the string. It returns an iterator yielding match objects. From each match object, you can use the start() and end() methods to get the exact positions of the match.

Here’s an example:

import re

pattern = re.compile("at")
matches = pattern.finditer("cat, bat, rat, sat")
positions = [(match.start(), match.end()) for match in matches]
print(positions)

Output:

[(1, 3), (6, 8), (11, 13), (16, 18)]

This code snippet compiles the regular expression for “at” and finds all iterations that match the pattern in the given string. Then, it constructs a list of tuple positions containing the start and end indices of each match.

Method 2: Using re.find() with a loop

If re.find() was an actual function (Please note, Python’s ‘re’ module doesn’t have a function called ‘re.find()’, it might be an indivual’s misconception of ‘find’ attribute in strings), it could hypothetically be used in a loop to locate all matches by updating the search position after each find. After finding a match, the start index could be updated to just after the last match to continue searching.

Here’s an example:

# Hypothetical code assuming re.find() exists which it does NOT.
import re

positions = []
pattern = "at"
string = "cat, bat, rat, sat"
start = 0

while True:
    match = re.find(pattern, string, start)
    if match:
        positions.append((match.start(), match.end()))
        start = match.end()
    else:
        break

print(positions)

Output (hypothetical):

[(1, 3), (6, 8), (11, 13), (16, 18)]

This code snippet would hypothetically update the search position and append the start and end positions of each match to the list until there are no more matches.

Method 3: Using re.search() in a loop

re.search() finds the first match of a pattern in a string. By repeatedly calling re.search() on the updated substring (excluding the previous match), you can find the indices of all matches.

Here’s an example:

import re

pattern = re.compile("at")
positions = []
string = "cat, bat, rat, sat"
start = 0

while True:
    match = pattern.search(string, start)
    if match:
        positions.append((match.start(), match.end()))
        start = match.end()
    else:
        break

print(positions)

Output:

[(1, 3), (6, 8), (11, 13), (16, 18)]

This code searches for the pattern “at” and updates the start index after each find. The loop breaks when no more matches are found. The positions are accumulated in a list.

Method 4: Using re.findall() with string slicing

The re.findall() function finds all substrings where the regex matches, but it doesn’t provide their positions. By slicing the string at each match index and searching again, one can keep track of the overall index to find the positions. Please note, you’d have to replicate some functionality of matching to get the actual index, so this approach is not straightforward.

Here’s an example:

# PLEASE NOTE: This method FINDS all occurrences but does NOT provide their exact positions.
import re

pattern = re.compile("at")
string = "cat, bat, rat, sat"
matches = pattern.findall(string)
print(matches)

Output:

['at', 'at', 'at', 'at']

This code snippet finds all occurrences of “at” in the given string. However, it does not provide their positions in the original string, which is the goal of this article.

Bonus One-Liner Method 5: Using List Comprehension with re.finditer()

By combining list comprehension with re.finditer(), you can perform the search and obtain the positions in a single, efficient line of code. This method offers a concise way to get the start and end indices of all matches.

Here’s an example:

import re

positions = [(m.start(), m.end()) for m in re.finditer("at", "cat, bat, rat, sat")]
print(positions)

Output:

[(1, 3), (6, 8), (11, 13), (16, 18)]

The one-liner creates a list of tuples with position indices directly from the iterator returned by re.finditer().

Summary/Discussion

  • Method 1: Using re.finditer(). Strengths: Directly provides match objects that can be used to find positions. Weaknesses: Slightly more verbose than a one-liner approach.
  • Method 2: Using re.find() with a loop. Strengths: Iterative, clear logic. Weaknesses: The function re.find() does not actually exist in Python’s ‘re’ module, and this method is purely hypothetical.
  • Method 3: Using re.search() in a loop. Strengths: Reliable, uses actual Python ‘re’ method functionality to search repeatedly. Weaknesses: May not be as efficient as re.finditer().
  • Method 4: Using re.findall() with string slicing. Strengths: Finds all occurrences easily. Weaknesses: Does not provide the positions, thus failing to meet the article’s goal.
  • Bonus Method 5: One-liner using list comprehension with re.finditer(). Strengths: Elegant and efficient. Weaknesses: May be less readable for beginners.