Regex Matches Digit: Uncovering the True Nature of "\d"

In Python's regex world, the enigmatic character class "\d" has raised questions about its precise matching behavior. Let's delve into its cryptic nature.

In the string "123", "\d" captures both "1" and "3" but curiously excludes "2". This peculiar behavior stems from a subtle nuance in the definition of a "digit."

Python's "\d" does not exclusively match digits from the Western numeric system (i.e., "0-9"). It also encompasses an extended range of Unicode character classes within the category of "Digit" or "Nd." These include, for instance, Eastern Arabic numerals such as "٠" and "١".

Thus, when matching digits in Python regex expressions, it's crucial to recognize that "\d" casts a wider net than [0-9]. This nuanced understanding unravels the mystery of why, in a sequence like "12345," only odd-ordered digits (i.e., "1", "3", and "5") align with "\d" matches, leaving even-ordered digits ("2" and "4") untouched.