Extract text from tag content using regular expression

[buran]

import requests
from bs4 import BeautifulSoup

url = 'http://www.amazon.com/s?k=9783319988412&ref=nb_sb_noss'

headers = {
    'User-Agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0',
    'Accept': 'text/html,*/*',
    'Accept-Language': 'en,en-US;q=0.7,en;q=0.3',
    'X-Requested-With': 'XMLHttpRequest',
    'Connection': 'keep-alive'}

resp = requests.get(url, headers=headers)
soup = BeautifulSoup(resp.text, 'lxml')

# using find
a = soup.find('a', {'class': 'a-link-normal a-text-normal'})
print(a.get('href'))

# using  css selectors
a1 = soup.select('a.a-link-normal.a-text-normal')
print(a1[0].get('href'))

Not sure why you get malformed selector error, but you probably don't want to select using part of the href attribute (would you change the selector for every page?)