You need to escape the backslash before the "3" Even if the backslash is in the quotes it will NOT be escaped.
If you're programitically inserting the value in the function, then I suggest performing another .replace and replace the "\" with a "-"

That should convert the string to "21-3-90" and if you need to break it down further, you can replace the "-" to a " " as you originally intended.

akhter@uf8b156e44b21553641ed:~/PycharmProjects/untitled2$ python3.2
Python 3.2.3 (default, Feb 27 2014, 21:31:18) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> def process(date):
...     date = date.replace("\\", " ")
...     print(date)
... 
>>> process("21\\3\\90")
21 3 90
>>> process("13\\4\\90")
13 4 90
>>> 

akhter@uf8b156e44b21553641ed:~/PycharmProjects/untitled2$ python2.7
Python 2.7.3 (default, Dec 18 2014, 19:10:20) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> def process(date):
...     date = date.replace("\\", " ")
...     print(date)
... 
>>> process("21\\3\\90")
21 3 90
>>> 
akhter@uf8b156e44b21553641ed:~/PycharmProjects/untitled2$ python3.2
Python 3.2.3 (default, Feb 27 2014, 21:31:18) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> def process(date):
...     date = date.replace('\\', ' ')
...     print(date)
... 
>>> process("21\\3\\90")
21 3 90
>>> 

r/learnpython is probably a better subreddit for these kinds of questions.

Having said that, your first example actually works, try:

print("apple".replace('l', '\\'))

We need to specify that we want to replace a string that contains a single backslash. We cannot write that as "\", because the backslash is escaping the intended closing double-quote. We also cannot use a raw string literal for this: r"\" does not work.

Instead, we simply escape the backslash using another backslash:

result = string.replace("\\","")

Assuming a 'tag' can also occur at the very beginning of your string, and avoid selecting false positives, maybe you could use:

\s?(?<!\S)\\[a-z\d]+

And replace with nothing. See an online demo.

• \s? - Optionally match a whitespace character (if a tag is mid-string and therefor preceded by a space);
• (?<!\S) - Assert position is not preceded by a non-whitespace character (to allow a position at the start of your input);
• \\ - A literal backslash.
• [a-z\d]+ - 1+ (Greedy) Characters as per given class.

You're getting what you want. It just doesn't look that way in the REPL:

>>> 'asdf hjkl'.replace(' ', '\s')[4]
'\\'

As you can see, that's one character, not two.

Try printing it:

>>> print 'asdf hjkl'.replace(' ', '\s')
asdf\shjkl

No need to use str.replace or string.replace here, just convert that string to a raw string:

>>> strs = r"C:\Users\Josh\Desktop\20130216"
           ^
           |
       notice the 'r'

Below is the repr version of the above string, that's why you're seeing \\ here. But, in fact the actual string contains just '\' not \\.

>>> strs
'C:\\Users\\Josh\\Desktop\\20130216'

>>> s = r"f\o"
>>> s            #repr representation
'f\\o'
>>> len(s)   #length is 3, as there's only one `'\'`
3

But when you're going to print this string you'll not get '\\' in the output.

>>> print strs
C:\Users\Josh\Desktop\20130216

If you want the string to show '\\' during print then use str.replace:

>>> new_strs = strs.replace('\\','\\\\')
>>> print new_strs
C:\\Users\\Josh\\Desktop\\20130216

repr version will now show \\\\:

>>> new_strs
'C:\\\\Users\\\\Josh\\\\Desktop\\\\20130216'

Elaborating this answer, with pathlib you can use the as_posix method:

>>> import pathlib
>>> p = pathlib.PureWindowsPath(r'\dir\anotherdir\foodir\more')
>>> print(p)    
\dir\anotherdir\foodir\more
>>> print(p.as_posix())
/dir/anotherdir/foodir/more
>>> str(p)
'\\dir\\anotherdir\\foodir\\more'
>>> str(p.as_posix())
'/dir/anotherdir/foodir/more'

Your \ is hiding the closing " so you need to use \\ - but python shouldn't care about the slashes in the path.

You can use the string .replace() method along with rawstring.

Python 2:

>>> print r'pictures\12761_1.jpg'.replace("\\", "/")
pictures/12761_1.jpg

Python 3:

>>> print(r'pictures\12761_1.jpg'.replace("\\", "/"))
pictures/12761_1.jpg

There are two things to notice here:

• Firstly to read the text as a drawstring by putting r before the string. If you don't give that, there will be a Unicode error here.
• And also that there were two backslashes given inside the replace method's first argument. The reason for that is that backslash is a literal used with other letters to work as an escape sequence. Now you might wonder what is an escape sequence. So an escape sequence is a sequence of characters that doesn't represent itself when used inside string literal or character. It is composed of two or more characters starting with a backslash. Like '\n' represents a newline and similarly there are many. So to escape backslash itself which is usually an initiation of an escape sequence, we use another backslash to escape it.

I know the second part is bit confusing but I hope it made some sense.