If you're new to REG(gular) EX(pressions) you learn about them at Python Docs. Or, if you want a gentler introduction, you can check out the HOWTO. They use Perl-style syntax.

Regex

The expression that you need is .*?\[(.*)\].*. The group that you want will be \1.
- .*?: . matches any character but a newline. * is a meta-character and means Repeat this 0 or more times. ? makes the * non-greedy, i.e., . will match up as few chars as possible before hitting a '['.
- \[: \ escapes special meta-characters, which in this case, is [. If we didn't do that, [ would do something very weird instead.
- (.*): Parenthesis 'groups' whatever is inside it and you can later retrieve the groups by their numeric IDs or names (if they're given one).
- \].*: You should know enough by now to know what this means.

Implementation

First, import the re module -- it's not a built-in -- to where-ever you want to use the expression.

Then, use re.search(regex_pattern, string_to_be_tested) to search for the pattern in the string to be tested. This will return a MatchObject which you can store to a temporary variable. You should then call it's group() method and pass 1 as an argument (to see the 'Group 1' we captured using parenthesis earlier). I should now look like:

>>> import re
>>> pat = r'.*?\[(.*)].*'             #See Note at the bottom of the answer
>>> s = "foobar['infoNeededHere']ddd"
>>> match = re.search(pat, s)
>>> match.group(1)
"'infoNeededHere'"

An Alternative

You can also use findall() to find all the non-overlapping matches by modifying the regex to (?>=\[).+?(?=\]).
- (?<=\[): (?<=) is called a look-behind assertion and checks for an expression preceding the actual match.
- .+?: + is just like * except that it matches one or more repititions. It is made non-greedy by ?.
- (?=\]): (?=) is a look-ahead assertion and checks for an expression following the match w/o capturing it.
Your code should now look like:

>>> import re
>>> pat = r'(?<=\[).+?(?=\])'  #See Note at the bottom of the answer
>>> s = "foobar['infoNeededHere']ddd[andHere] [andOverHereToo[]"
>>> re.findall(pat, s)
["'infoNeededHere'", 'andHere', 'andOverHereToo['] 

Note: Always use raw Python strings by adding an 'r' before the string (E.g.: r'blah blah blah').

10x for reading! I wrote this answer when there were no accepted ones yet, but by the time I finished it, 2 ore came up and one got accepted. :( x<

Let me know if this is what you are looking for:

import re

def smallest_between_two(a, b, text):
    return min(re.findall(re.escape(a)+"(.*?)"+re.escape(b),text), key=len)

print(smallest_between_two(' ', '(', 'def test()'))
print(smallest_between_two('[', ']', '[this one][not this one]'))
print(smallest_between_two('paste ', '/', '@paste "game_01/01"'))

Output:

test
this one
"game_01

To add an explanation to what this does:

re.findall():

Return all non-overlapping matches of pattern in string, as a list of strings

re.escape()

Escape all the characters in pattern except ASCII letters and numbers. This is useful if you want to match an arbitrary literal string that may have regular expression metacharacters in it

(.*?)

.*? matches any character (except for line terminators)

*? Quantifier — Matches between zero and unlimited times, as few times as possible, expanding as needed (lazy)

So our regular expression matches any character (not including line terminators) between two arbitrary escaped strings, and then returns the shortest length string from the list that re.findall() returns.

Use re.search

>>> import re
>>> s = 'Part 1. Part 2. Part 3 then more text'
>>> re.search(r'Part 1\.(.*?)Part 3', s).group(1)
' Part 2. '
>>> re.search(r'Part 1(.*?)Part 3', s).group(1)
'. Part 2. '

Or use re.findall, if there are more than one occurances.

If the string contains only one instance, use re.search() instead:

>>> import re
>>> s = "api('randomkey123xyz987', 'key', 'text')"
>>> match = re.search(r"api\('([^']*)'", s).group(1)
>>> print match
randomkey123xyz987

Use this regex:

period_1_(.*)\.ssa

For example, in Perl you would extract it like this:

my ($substr) = ($string =~ /period_1_(.*)\.ssa/);

For Python, use this code:

m = re.match(r"period_1_(.*)\.ssa", my_long_string)
print m.group(1)

Last print will print string you are looking for (if there is a match).

Tweeky way!

>>> char1 = '('
>>> char2 = ')'
>>> mystr = "mystring(123234sample)"
>>> print mystr[mystr.find(char1)+1 : mystr.find(char2)]
123234sample