How can I replace all occurrences of a substring using regex?

re.sub(r'\bfoo\b', 'bar', s)

Here, the \b defines the word boundaries - positions between a word character (\w) and a non-word character - exactly what you have matching for foo inside the sdfjoiweng%@$foo$fsoifjoi string. Works for me:

In [1]: import re

In [2]:  s = 'sdfjoiweng%@$foo$fsoifjoi'

In [3]: re.sub(r'\bfoo\b', 'bar', s)
Out[3]: 'sdfjoiweng%@$bar$fsoifjoi'

You can use replace function directly instead of using regex.

>>> s = 'sdfjoiweng%@$foo$fsoifjoifoo'
>>>
>>> s.replace("foo","bar")
'sdfjoiweng%@$bar$fsoifjoibar'
>>>
>>>

You can replace it directly:

>>> import re
>>> s = 'sdfjoiweng%@$foo$fsoifjoi'
>>> print(re.sub('foo','bar',s))
sdfjoiweng%@$bar$fsoifjoi

It will also work for more occurrences of foo like below:

>>> s = 'sdfjoiweng%@$foo$fsoifoojoi'
>>> print(re.sub('foo','bar',s))
sdfjoiweng%@$bar$fsoibarjoi

If you want to replace only the 1st occurrence of foo and not all the foo occurrences in the string then alecxe's answer does exactly that.