terse

@echo off&setlocal enabledelayedexpansion
chcp 65001>nul
for /f "tokens=1,2*delims=.:" %%i in ('findstr /ibn "[0-9][0-9]*\." test.txt') do set "_%%i=%%k"
(for /f "tokens=1*delims=:" %%i in ('findstr /in .* test.txt') do (
      if defined _%%i (
         for /f %%a in ("!_%%i!") do for /f "tokens=1*delims=0123456789" %%b in ("%%a") do (
              if "%%b%%c" == "第章" (echo !_%%i!)else echo %%j
         )
      )else  echo;%%j
))
pause
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terse

z这个直接修改文件，运行前备份原文件

@echo off 
powershell -c "$enc=[Text.Encoding]::UTF8;[IO.Directory]::EnumerateFiles($pwd,'*.txt').foreach({[IO.File]::WriteAllLines($_,([IO.File]::ReadAllLines($_,$enc) -replace '^\d+\.(?=第\d+章\s+.*)', ''),$enc) })"
pause
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