Why Does `re.findall` Fail to Find Numbers in a String When `re.search` Succeeds?

re.findall Behaves Unpredictably

With the original string and pattern as defined below:

s = r'abc123d, hello 3.1415926, this is my book'
pattern = r'-?[0-9]+(\.[0-9]*)?|-?\.[0-9]+'

re.search accurately finds a match, while re.findall yields an empty list. This behavior, which contradicts the expected output of ['123', '3.1415926'], raises the question of why re.findall does not produce this result.

Understanding the Issue

Two key considerations are at play here:

1. Capturing Groups: re.findall extracts and returns only captured text when the regex pattern contains capturing groups.
2. Literal Backslash Matching: The \ within the pattern attempts to match a literal , rather than the intended . character.

Regex Modification

To address this, we can modify the pattern to remove redundant capturing groups and match numbers correctly:

pattern = r'-?\d*\.?\d+'

This pattern will match:

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

Results

Using this modified pattern, re.findall will produce the expected output:

import re
L = re.findall(pattern, s)
print(L)  # Output: ['123', '3.1415926']

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