Why Does `re.findall()` Return an Empty List When `re.search()` Finds a Match?

re.findall Behavior

The re.findall() function can be confusing if it doesn't return the expected results when matching a string. Let's explore the reasons behind its behavior in a specific case.

Problem Statement

Consider the following source string:

s = r'abc123d, hello 3.1415926, this is my book'

And the following pattern:

pattern = r'-?[0-9]+(\.[0-9]*)?|-?\.[0-9]+'

With re.search, we get the correct result:

m = re.search(pattern, s)
print(m)  # <_sre.SRE_Match object; span=(3, 6), match='123'>

However, re.findall returns an empty list:

L = re.findall(pattern, s)
print(L)  # []

Understanding the Issue

There are two key aspects to consider:

1. Empty Match Capture Groups: re.findall returns captured texts from the match object, but in this pattern, there are no capturing groups. As a result, it returns empty strings.
2. Character Escaping: The \. in the pattern matches two characters: and any character except newline. This is not intended for matching numeric values.

Solution

To correctly match numeric values, use the following pattern instead:

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

This pattern matches:

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

With this corrected pattern, re.findall will return the expected list:

['123', '3.1415926']

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