Expand currency sign with moving words inside string

You could make a dictionary mapping currency symbol to name, and then generate the regexes. Note that these regexes will only work for something in the form of a number and then a word.

import re
CURRENCIES = {
    r"\$": "dollars", # Note the slash; $ is a special regex character
    "€": "euros",
    "£": "pounds",
}
REGEXES = []
for symbol, name in CURRENCIES.items():
    REGEXES.append((re.compile(rf"{symbol}(\d+ [^\W\d_]+)"), rf"\1 {name}"))
text = """around $98 million last week
above $10 million next month
about €5 billion past year
after £1 billion this day"""
for regex, replacement in REGEXES:
    text = regex.sub(replacement, text)

You probably have more cases than this so the regular expression may be too specific if you have more cases, but re.sub can be used with a function to process each match and make the correct replacement. Below solves for the cases provided:

import re

text = '''\
around $98 million last week
above $10 million next month
about €5 billion past year
after £1 billion this day
'''

currency_name = {'$':'dollars', '€':'euros', '£':'pounds'}

def replacement(match):
    # group 2 is the digits and million/billon,
    # tack on the currency type afterward using a lookup dictionary
    return f'{match.group(2)} {currency_name[match.group(1)]}'

# capture the symbol followed by digits and million/billion
print(re.sub(r'([$€£])(\d+ [mb]illion)\b', replacement, text))

Output:

around 98 million dollars last week
above 10 million dollars next month
about 5 billion euros past year
after 1 billion pounds this day

It's useful in a case like this to remember that re.sub can accept a lambda rather than just a string.

The following requires Python 3.8+ for the := operator.

s = "about €5 billion past year"

re.sub(r'([€$])(\d+)\s+([mb]illion)', 
       lambda m: f"{(g := m.groups())[1]} {g[2]} {'euros' if g[0] == '€' else 'dollars'}", 
       s)
# 'about 5 billion euros past year'