terse

也来个纯P，尽量处理特殊字符吧

@echo off
for /f "delims=" %%f in ('dir/b/s/a-d *.txt') do (
     (for /f "usebackq tokens=1* delims=#" %%i in ("%%f") do (
           set "str=%%i"
           set "s=%%j"
           if "%%j" neq "" (
               call :loop
               setlocal enabledelayedexpansion
               echo !str!
               endlocal
          ) else echo %%i
     ))>"New_%%~nf.tx"
)
pause
exit
:loop
if "%s%" == ""  goto :eof
for /f "tokens=2* delims=#" %%i in ("%s%") do (
     set "t=%%i"
     set "s=%%j"
     setlocal enabledelayedexpansion
     if "!t: =!" neq "" set "str=!str!!t!"
     for /f "delims=" %%a in ("!str!") do endlocal&set "str=%%a"
     call :loop
)
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第三方gawk 处理子目录 要加FOR

gawk "{gsub(/#[^#]*#/, \"\"); print >>FILENAME\"NEW_tx\"}" *.txt 
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aloha20200628

订正了15楼代码。其中jscript和python脚本因均以文件而非行作为处理单位，故可有效解析定界符内容被断行的情况。

Five66

来个偷懒的，保存为ansi编码的bat
仅处理当前路径中文件夹里的文本文件
要求文本文件编码为gbk，文件名不要有英文百分号或936代码页无法显示的字符，还有文本中如果包含^可能会被吞
改了下，已最大程度支持特殊字符（大概）

@echo off&chcp 936 >nul

for /d %%a in (*) do (
for /f "delims=" %%b in ('dir /b /s /a-d "%%a\*.txt"') do (
setlocal
for /f "delims=" %%c in ('findstr /n .* "%%b"') do (
set "line=%%c"
setlocal enabledelayedexpansion
if "!line:*:=!" == "" (endlocal&echo,) else (
set "line=!line:*:=!"
for /f "delims=" %%i in ("$!line:#=^#!") do endlocal&set "line=%%i"
set o=
call :aaa
)
)
endlocal
)>"%%~dpb_new_%%~nxb"
)
pause&exit/b

:aaa
setlocal enabledelayedexpansion
set "line=!line:~1!"
for /f "tokens=1* delims=^" %%x in ("$!line!") do (
endlocal&set line=&set "tk1=%%x"&set "tk2=%%y"
)
setlocal enabledelayedexpansion
set "tk1=!tk1:~1!"
if "!tk1:~0,1!" == "#" (
if "!tk2:~0,1!" == "#" (set "line=!tk2:~1!") else set "o=!o!!tk1!"&set "line=!tk2!"
) else (
set "o=!o!!tk1!"
set "line=!tk2!"
)
if defined line (
for /f "delims=" %%i in ("$!line!") do (
for /f "delims=" %%j in (""!o!"") do endlocal&set "line=%%i"&set "o=%%~j"
)
goto :aaa
) else echo,!o!&endlocal
goto :eof
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aloha20200628

一楼示例中没有那些 ^&!%()<\|/> 等特殊字符作祟，故15楼的纯P代码可以应付，当然 jscript/python/sed 等解决方案可以对此无惧。
若真要考虑纯P解法能处理上述特殊字符，还须订正其中的代码(包括字符串长度获取函数)如下》

按一楼示例原意顺便改写一个包含特殊字符的测试样本 test.txt：
'~^&!%(#将被删除#)<\|/>
'~^&!%()<\|/>#将被删除#
#将被删除#'~^&!%()<\|/>

@echo off
for /f "delims=" %%F in ('dir/b/s/a-d *.txt') do (
	(for /f "usebackq delims=" %%s in ("%%~F") do (
		set s=%%~s
		setlocal enabledelayedexpansion
		set "cut=-1" &set "_s=" &(call :_strLen s sL)
		for /L %%i in (0,1,!sL!) do (
			set "c=!s:~%%i,1!"
			if "!c!" neq "#" (if !cut! equ -1 set "_s=!_s!!c!") else (set/a "cut=0-!cut!")
		)
		(echo,!_s!)
		endlocal
	))>"%%~F.new"
)
endlocal &exit/b

:_strLen //获取字符串长度(须先开启变量延迟可保全^!等特殊字符) %1=被测字符串变量名 %2=返回值变量名
	set "_str=_!%~1!" &set "Len=0"
	for %%n in (4096 2048 1024 512 256 128 64 32 16 8 4 2 1) do (
		if "!_str:~%%n,1!" neq "" (set/a "Len+=%%n"&set "_str=!_str:~%%n!")
	)
	set/a "%~2=!Len!-1" &exit/b
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