13. String Functions (`.str` accessor)

s = pd.Series(['  Alice  ', 'BOB', 'charlie99'])
s.str.strip()                  # trim whitespace
s.str.lower()                  # lowercase
s.str.contains('bob', case=False)   # boolean mask
s.str.replace('9', '', regex=True)
s.str.startswith('A')
s.str.endswith('e')
s.str.len()
s.str.split('-')               # returns lists
s.str.extract(r'(\d+)')        # regex capture group -> DataFrame
s.str.cat(sep=', ')            # join
'a,b,c'.split(',')             # plain python (for reference)

💡 Tip: str.split('-', expand=True) splits into multiple columns instead of a column of lists.

⚠️ Common Mistake: .str methods return NaN for missing values and skip them — check for NaN before/after.

⭐ Interview Question: How to extract a pattern from text? df['col'].str.extract(r'(\d{4})') captures the first regex group into a new column.