Why Doesn't `re.findall` Return Overlapping Regex Matches, and How Can Lookahead Assertions Solve This?

Uncovering Overlapping Regex Matches: Dive into Lookahead Assertions

Problem:
When using re.findall to match a regular expression pattern, why does it not retrieve all overlapping matches? For instance, in the string "hello," why does the regex r'ww' only match "he" and "ll" but not "el" and "lo"?

Answer:
By default, re.findall does not yield overlapping matches. To achieve this, employ a lookahead assertion, a powerful regex feature.

Solution:

# Using a lookahead assertion
matches = re.findall(r'(?=(\w\w))', 'hello')

# Output: ['he', 'el', 'll', 'lo']

The (?=...) construct in the regex is a lookahead assertion. It matches if the specified pattern appears immediately after the current position, but it does not consume any characters from the string. In this case, it identifies all two-character sequences ("ww") in "hello" without consuming any characters.

Explanation:

• The parenthetical expression (ww) defines the two-character pattern to match.
• (?=) precedes the pattern, indicating a lookahead assertion.
• The regular expression引擎 moves the cursor along "hello" and continuously checks if the next two characters match the pattern "ww."
• If they do, it records the current position as a match.
• This process continues, resulting in the detection of all overlapping matches: "he," "el," "ll," and "lo."

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