Add str.extract

[Uwe L. Korn]

• ✔️ pandas function
• ❌ no Python function
• ❌ needs a regular expression library
• ✔️ no need for a Unicode database
• ❌ cannot pre-compute output size

Pseudo-Code:

Inputs: pat, expand

builder = StringBuilder()
for row in rows:
    if isnull(row)
        builder.addnull()
    else:
        # TODO …