Add `regex` scalar function to extract values from strings

[norberttech]

This is a use case from a question we got on a discord server:

Input:

<rss xmlns:g="http://base.google.com/ns/1.0">
<channel>
    <item>
        <g:price>123 EUR</g:price>
    </item>
    <item>
        <g:price>154.12 USD</g:price>
    </item>
</channel>
</rss>

expected output:

(price and currency in separate columns)

+--------------+--------+----------+
| price_string |  price | currency |
+--------------+--------+----------+
|      123 EUR | 123.00 |      EUR |
+--------------+--------+----------+
1 rows
+--------------+--------+----------+
| price_string |  price | currency |
+--------------+--------+----------+
|   154.12 USD | 154.12 |      USD |
+--------------+--------+----------+

Currently, it can be achieved by replacing currency/price with empty strings:

df()
    ->read(from_xml(__DIR__ . '/file.xml', 'rss/channel/item'))
    ->withEntry('price_string', ref('node')->xpath('g:price')->domElementValue())
    ->withEntry('price', ref('price_string')->regexReplace(lit('/(A-Z{3})/'), lit(''))->cast('float')->round(lit(2)))
    ->withEntry('currency', ref('price_string')->regexReplace(lit('/\d+(?:\.\d+)?/'), lit('')))
    ->drop('node')
    ->write(to_output(false))
    ->run();

Because we don't have a scalar function that would extract a match from regexp.

I think we can add something similar to:

ref('name')->regex(lit("pattern"), match: lit(0))

Regex - ScalarFunction similar to PregMatch function but instead of confirming that the string matches a pattern, it would let us return given match (or multiple matches maybe)