How can I replace the first occurrence of a character in every word?

I would do a regex replacement on the following pattern:

@(@*)

And then just replace with the first capture group, which is all continous @ symbols, minus one.

This should capture every @ occurring at the start of each word, be that word at the beginning, middle, or end of the string.

inp = "hello @jon i am @@here or @@@there and want some@thing in '@here"
out = re.sub(r"@(@*)", '\\1', inp)
print(out)

This prints:

hello jon i am @here or @@there and want something in 'here

How about using replace('@', '', 1) in a generator expression?

string = 'hello @jon i am @@here or @@@there and want some@thing in "@here"'
result = ' '.join(s.replace('@', '', 1) for s in string.split(' '))

# output: hello jon i am @here or @@there and want something in "here"

The int value of 1 is the optional count argument.

str.replace(old, new[, count])

Return a copy of the string with all occurrences of substring old replaced by new. If the optional argument count is given, only the first count occurrences are replaced.

You can use re.sub like this:

import re

s = "hello @jon i am @@here or @@@there and want some@thing in '@here"
s = re.sub('@(\w)', r'\1', s)
print(s)

That will result in:

"hello jon i am @here or @@there and want something in 'here"

And here is a proof of concept:

>>> import re
>>> s = "hello @jon i am @@here or @@@there and want some@thing in '@here"
>>> re.sub('@(\w)', r'\1', s)
"hello jon i am @here or @@there and want something in 'here"
>>> 

Was pondering for cases what if only the last char is @ and you don't want to remove it, or you have specific allowed starting chars, came up with this:

>>> ' '.join([s_.replace('@', '', 1) if s_[0] in ["'", "@"] else s_ for s_ in s.split()])
"hello jon i am @here or @@there and want some@thing in 'here"

Or, suppose you want to replace @ only if it is in first n characters

>>> ' '.join([s_.replace('@', '', 1) if s_.find('@') in range(2) else s_ for s_ in s.split()])
"hello jon i am @here or @@there and want some@thing in 'here"