New REPL `input` bad cursor position when a backspace char `\b` is in the prompt

Bug report

Bug description:

New REPL does not position the cursor correctly, if the prompt includes backspace character (\b = \x08)

# Basic REPL shows correct behavior
 ~$ PYTHON_BASIC_REPL=1 python3
Python 3.15.0a7+ (heads/main:5197ecb, Mar 10 2026, 20:55:52) [GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> input("Question? _\b")
Question? █
Question? Yes█
'Yes'
>>> 

# New REPL misposition the cursor. 
 ~$ python3
Python 3.15.0a7+ (heads/main:5197ecb, Mar 10 2026, 20:55:52) [GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> input("Question? _\b")
Question? _ █
Question? Yes  █
'Yes'
>>> 

I've tested code below, and it fixes the issue:

--- ~/.pyenv/versions/3.15-dev/lib/python3.15/_pyrepl/utils.py
+++ ~/.pyenv/versions/3.15-dev/lib/python3.15/_pyrepl/utils.py
@@ -82,7 +82,9 @@
     # remove lengths of any escape sequences
     sequence = ANSI_ESCAPE_SEQUENCE.findall(s)
     ctrl_z_cnt = s.count("\x1a")
-    return length - sum(len(i) for i in sequence) + ctrl_z_cnt
+    # remove length of backspaces x 2
+    backspace = s.count("\b")
+    return length - sum(len(i) for i in sequence) + ctrl_z_cnt - 2 * backspace
 
 
 def unbracket(s: str, including_content: bool = False) -> str:

CPython versions tested on:

3.15

Operating systems tested on:

Linux

Linked PRs

• gh-150488

使用 input("Question? _\b") 重現新 REPL 中的游標位置問題，然後檢查 _pyrepl/utils.py 中顯示長度計算附近的程式碼。將其行為與 gh-150488 中連結的工作進行比較；當提示字串和輸入的文字讓游標停留在與基本 REPL 相同的位置時，即表示完成。