how to use regular expression to match the string included two backslash

Question

I have to write a regular expression refer to a string.I need to get the part between "$u" and the final "$",also I need match the part before "$u" Now I wrote the regular expression as follow, but it can not work

a='=856  \\$uhttp://sfx-852cuh.hosted$'
#Two backslash may replace by other characters:a='=856  aa$uhttp://sfx-852cuh.hosted$'
result=re.search('=\w{3}\s{2}\S{2}\$u(.*)\$', a)
target_str=result.group(1)

Answer 1

Replace \\S{2} with \\S{1,2} if you expect 1 or 2 non-whitespace chars before $u and do not use a variable with str name:

import re
a='=856  \\$uhttp://sfx-852cuh.hosted$'
result=re.search(r'=\w{3}\s{2}\S{1,2}\$u(.*)\$', a)
expected_value = ''
if result:
    expected_value = result.group(1)
print(expected_value)   

See the Python demo

Answer 2

A side note - '\\' in a string is the representation of a single '\\', as it is the start of a special character.

Anyway you can make life easier - if you're guaranteed only a single "$u":

a='=856  \\$uhttp://sfx-852cuh.hosted$'
#Two backslash may replace by other characters:a='=856  aa$uhttp://sfx-852cuh.hosted$'
result=re.search('(.*)\$u(.*)\$', a)
NOT_str=result.group(2)

Group 1 contains the stuff before the $u .