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How I can use regex to remove repeated characters from string

I have a string as follows where I tried to remove similar consecutive characters.
import re
input = "abccbcbbb";
for i in input :
input = re.sub("(.)\\1+", "",input);
print(input)
Now I need to let the user specify the value of k.
I am using the following python code to do it, but I got the error message TypeError: can only concatenate str (not "int") to str
import re
input = "abccbcbbb";
k=3
for i in input :
input= re.sub("(.)\\1+{"+(k-1)+"}", "",input)
print(input)

The for i in input : does not do what you need. i is each character in the input string, and your re.sub is supposed to take the whole input as a char sequence.
If you plan to match a specific amount of chars you should get rid of the + quantifier after \1. The limiting {min,} / {min,max} quantifier should be placed right after the pattern it modifies.
Also, it is more convenient to use raw string literals when defining regexps.
You can use
import re
input_text = "abccbcbbb";
k=3
input_text = re.sub(fr"(.)\1{{{k-1}}}", "", input_text)
print(input_text)
# => abccbc
See this Python demo.
The fr"(.)\1{{{k-1}}}" raw f-string literal will translate into (.)\1{2} pattern. In f-strings, you need to double curly braces to denote a literal curly brace and you needn't escape \1 again since it is a raw string literal.

If I were you, I would prefer to do it like suggested before. But since I've already spend time on answering this question here is my handmade solution.
The pattern described below creates a named group named "letter". This group updates iterative, so firstly it is a, then b, etc. Then it looks ahead for all the repetitions of the group "letter" (which updates for each letter).
So it finds all groups of repeated letters and replaces them with empty string.
import re
input = 'abccbcbbb'
result = 'abcbcb'
pattern = r'(?P<letter>[a-z])(?=(?P=letter)+)'
substituted = re.sub(pattern, '', input)
assert substituted == result

Just to make sure I have the question correct you mean to turn "abccbcbbb" into "abcbcb" only removing sequential duplicate characters. Is there a reason you need to use regex? you could likely do a simple list comprehension. I mean this is a really cut and dirty way to do it but you could just put
input = "abccbcbbb"
input = list(input)
previous = input.pop(0)
result = [previous]
for letter in input:
if letter != previous : result += letter
previous = letter
result = "".join(result)
and with a method like this, you could make it easier to read and faster with a bit of modification id assume.

Remove Characters From A String Until A Specific Format is Reached

So I have the following strings and I have been trying to figure out how to manipulate them in such a way that I get a specific format.
string1-itd_jan2021-internal
string2itd_mar2021-space
string3itd_feb2021-internal
string4-itd_mar2021-moon
string5itd_jun2021-internal
string6-itd_feb2021-apollo
I want to be able to get rid of any of the last string so I am just left with the month and year, like below:
string1-itd_jan2021
string2itd_mar2021
string3itd_feb2021
string4-itd_mar2021
string5itd_jun2021
string6-itd_feb2021
I thought about using string.split on the - but then realized that for some strings this wouldn't work. I also thought about getting rid of a set amount of characters by putting it into a list and slicing but the end is varying characters length?
Is there anything I can do it with regex or any other python module?

Use str.rsplit with the appropriate maxsplit parameter:
s = s.rsplit("-", 1)[0]
You could also use str.split (even though this is clearly the worse choice):
s = "-".join(s.split("-")[:-1])
Or using regular expressions:
s = re.sub(r'-[^-]*$', '', s)
# "-[^-]*" a "-" followed by any number of non-"-"

With a regex:
import re
re.sub(r'([0-9]{4}).*$', r'\1', s)

Use re.sub like so:
import re
lines = '''string1-itd_jan2021-internal
string2itd_mar2021-space
string3itd_feb2021-internal
string4-itd_mar2021-moon
string5itd_jun2021-internal
string6-itd_feb2021-apollo'''
for old in lines.split('\n'):
new = re.sub(r'[-][^-]+$', '', old)
print('\t'.join([old, new]))
Prints:
string1-itd_jan2021-internal string1-itd_jan2021
string2itd_mar2021-space string2itd_mar2021
string3itd_feb2021-internal string3itd_feb2021
string4-itd_mar2021-moon string4-itd_mar2021
string5itd_jun2021-internal string5itd_jun2021
string6-itd_feb2021-apollo string6-itd_feb2021
Explanation:
r'[-][^-]+$' : Literal dash (-), followed by any character other than a dash ([^-]) repeated 1 or more times, followed by the end of the string ($).

Add spaces at the beginning of the print output in python

I'm wondering how Am I suppose to add 4 spaces at the beginnings of the print outputs with f-string or format in python?
This is what I use to print now:
print('{:<10}'.format('Constant'),'{:{width}.{prec}f}'.format(122, width=10, prec=3))
and my output look like this:
Constant 122.000
but what I want is to have 4 spaces before the constant in the output like:
( 4 spaces here )Constant 122.000
Any ideas? Thanks a lot!

You can use ' ' + string (as suggested), but a more robust approach could be:
string="Test String leading space to be added"
spaces_to_add = 4
string_length=len(string) + spaces_to_add # will be adding 4 extra spaces
string_revised=string.rjust(string_length)
result:
' Test String leading space to be added'

There's a couple ways you could do it:
If Constant is really an unchanging constant, why not just
print(f" {Constant}", ...)
before your other string?
With your current implementation, you are left-aligning to a width of 10 characters. If you swap that to right-align, like '{:>12}'.format('Constant') ("Constant" is 8 characters, 12 - 8 = 4 spaces) It will put 4 characters in front of the string.
Here's a Python f-string syntax cheat sheet I've used before:
https://myshell.co.uk/blog/2018/11/python-f-string-formatting-cheatsheet/
And the official docs: PEP 3101

Regex include the negative lookbehind

I'm trying to filter a string before passing it through eval in python. I want to limit it to math functions, but I'm not sure how to strip it with regex. Consider the following:
s = 'math.pi * 8'
I want that to basically translate to 'math.pi*8', stripped of spaces. I also want to strip any letters [A-Za-z] that are not followed by math\..
So if s = 'while(1): print "hello"', I want any executable part of it to be stripped:
s would ideally equal something like ():"" in that scenario (all letters gone, because they were not followed by math\..
Here's the regex I've tried:
(?<!math\.)[A-Za-z\s]+
and the python:
re.sub(r'(?<!math\.)[A-Za-z\s]+', r'', 'math.pi * 8')
But the result is '.p*8', because math. is not followed by math., and i is not followed by math..
How can I strip letters that are not in math and are not followed by math.?
What I ended up doing
I followed #Thomas's answer, but also stripped square brackets, spaces, and underscores from the string, in hopes that no python function can be executed other than through the math module:
s = re.sub(r'(\[.*?\]|\s+|_)', '', s)
s = eval(s, {
'__builtins__' : None,
'math' : math
})

As #Carl says in a comment, look at what lybniz does for something better. But even this is not enough!
The technique described at the link is the following:
print eval(raw_input(), {"__builtins__":None}, {'pi':math.pi})
But this doesn't prevent something like
([x for x in 1.0.__class__.__base__.__subclasses__()
if x.__name__ == 'catch_warnings'][0]()
)._module.__builtins__['__import__']('os').system('echo hi!')
Source: Several of Ned Batchelder's posts on sandboxing, see http://nedbatchelder.com/blog/201302/looking_for_python_3_builtins.html
edit: pointed out that we don't get square brackets or spaces, so:
1.0.__class__.__base__.__subclasses__().__getitem__(i)()._module.__builtins__.get('__import__')('os').system('echo hi')
where you just try a lot of values for i.

I want to remove '\' from a string in python [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
How to refer to “\” sign in python string
I've quite large string data in which I've to remove all characters other than A-Z,a-z and 0-9
I'm able to remove almost every character but '\' is a problem.
every other character is removed but '\' is making problem
def replace_all(text, dic):
for i, j in dic.iteritems():
text = text.replace(i, j)
return text
reps = {' ':'-','.':'-','"':'-',',':'-','/':'-',
'<':'-',';':'-',':':'-','*':'-','+':'-',
'=':'-','_':'-','?':'-','%':'-','!':'-',
'$':'-','(':'-',')':'-','\#':'-','[':'-',
']':'-','\&':'-','#':'-','\W':'-','\t':'-'}
x.name = x.name.lower()
x1 = replace_all(x.name,reps)

I've quite large string data in which I've to remove all characters other than A-Z,a-z and 0-9
In other words, you want to keep only those characters.
The string class already provides a test "is every character a letter or number?", called .isalnum(). So, we can just filter with that:
>>> filter(str.isalnum, 'foo-bar\\baz42')
'foobarbaz42'

If you have a string:
a = 'hi how \\are you'
you can remove it by doing:
a.replace('\\','')
>'hi how are you'
If you have a specific context where you are having trouble, I recommend posting a bit more detail.

birryee is correct, you need to escape the backslash with a second backslash.

to remove all characters other than A-Z, a-z and 0-9
Instead of trying to list all the characters you want to remove (that would take a long time), use a regular expression to specify those characters you wish to keep:
import re
text = re.sub('[^0-9A-Za-z]', '-', text)