Character Classification (Digit, Letter, Space, …)

Classification buckets each character: letter, digit, whitespace, punctuation, or other. Python’s str methods (isdigit, isalpha, isspace, isalnum) are thin wrappers around Unicode categories—know their semantics.

Why this shows up in the real world

Banking parsers classify IBAN characters into check digits vs country codes. Log scrapers skip whitespace tokens. Password rules require at least one “symbol” class.

Core idea (explained for students)

Build small predicates and compose: is_ident_start = lambda c: c.isalpha() or c=='_'. For ASCII-only, combine with c.isascii().

Try this in Python

def token_kind(ch: str) -> str:
    if ch.isspace():
        return "space"
    if ch.isdigit():
        return "digit"
    if ch.isalpha():
        return "letter"
    return "other"


print([token_kind(c) for c in "A 9#"])

Common mistakes

• isdecimal vs isdigit vs isnumeric—different sets in Unicode.
• Treating _ inconsistently between “word” and “identifier” rules.

Key takeaways

• Print a truth table for sample characters when learning.
• Wrap classifiers in named functions to keep parsers readable.