The following regular expression

[lilzz]

1) match = re.match(r"B(\d+)\[(\d+)\]$", bit)

what does "B(\d+)\[(\d+)\]$" means?

2) match = re.match(r"(.*?_)(\d+)(.*)", s[2])

"(.*?_)(\d+)(.*)"?

[ame]

Google still broken for you?

Knock yourself out:
http://www.zytrax.com/tech/web/regex.htm

[SonOfAMotherlessGoat]

/B(\d+)\[(\d+)\]$/
B matches the character B literally (case sensitive)
1st Capturing group (\d+)
\d+ match a digit [0-9]
Quantifier: + Between one and unlimited times, as many times as possible, giving back as needed [greedy]
\[ matches the character [ literally
2nd Capturing group (\d+)
\d+ match a digit [0-9]
Quantifier: + Between one and unlimited times, as many times as possible, giving back as needed [greedy]
\] matches the character ] literally
$ assert position at end of the string
MATCH INFORMATION

https://regex101.com/

[DougieLawson]

Grab a copy of http://www.weitz.de/regex-coach/ and install it on your Windows system. That lets you disassemble regexes and step through a regex versus a target string showing the matches as you go.