Why Does `re.findall` Return an Empty List While `re.search` Works Correctly with the Same Pattern and String in Python 3.4.3?

re.findall Behaves Anomalous

Dilemma:

In a Python script using Python 3.4.3, the re.search method provides the anticipated result when applied to a particular source string using a specified pattern. However, re.findall inexplicably returns an empty list for the same pattern and source string.

Exploration:

Curiously, the re.findall method behaves unexpectedly despite appearing to fulfill the requirements for extracting matches from a string based on a pattern.

Resolution:

Upon careful examination, it becomes evident that two factors contribute to this peculiar behavior:

• Pattern Composition:
  The provided pattern contains a contentious expression, r'.', which aims to match an escape character () followed by any non-newline character. However, within the context of an r'' string literal, this expression attempts to match a literal , resulting in incorrect behavior.
• findall Behavior:
  The re.findall method expects the pattern to contain capturing groups. If no capturing groups are present, it returns empty strings. The original pattern lacks capturing groups, rendering it unable to capture the desired matches.

Rectified Pattern:

To alleviate this issue and successfully extract matches, the following rectified pattern should be employed:

-?\d*\.?\d+

Explanation:

• -?: Optional minus sign
• d*: Optional digits
• .?: Optional decimal separator
• d : One or more digits

Demonstration:

Utilizing the modified pattern, the re.findall method now accurately retrieves the anticipated list of matches:

['123', '3.1415926']

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