FINDSTR

Use the FINDSTR command to search for a specific string in a file or files and send the specified lines to your output device.
FINDSTR was introduced in the Windows NT 4 Resource Kit and is now a native command in Windows 2000 and later.

Syntax:

FINDSTR
Searches for strings in files.

Use spaces to separate multiple search strings unless the argument is prefixed with /C.
For example FINDSTR "hello there" x.y searches for "hello" or "there" in file x.y.
FINDSTR /C:"hello there" x.y searches for "hello there" in file x.y.

Regular expression quick reference:

To learn more about regular expressions read Mastering Regular Expressions (2nd Edition) by Jeffrey E.F. Friedl.
More titles on regular expressions can be found here.

FINDSTR may not be available on every system your batch file targets: though native in Windows 2000 and later, it has to be bought and installed separately on Windows NT 4 systems.

The following code snippet can be used to detect if FINDSTR is available:

FINDSTR /? >NUL 2>&1
IF ERRORLEVEL 1 ECHO FINDSTR not available!

or:

FINDSTR /? >NUL 2>&1 || ECHO FINDSTR not available!

This trick has been used in ReadReg.bat to decide if FINDSTR or EGREP should be used.

Related Stuff:

• The best and most exhaustive explanation (as far as I know) of FINDSTR and its limitations: What are the undocumented features and limitations of the Windows FINDSTR command?
• Why does findstr not handle case properly (in some circumstances)?
• FINDSTR documentation on TechNet
• FINDSTR and EGREP can be used to filter on a line-by-line basis only, i.e. the search pattern must match within a single line.
  To allow regex search patterns spanning multiple lines, use my RxGrep.exe.
  An example:
  
  ROBOCOPY D:\sourcedir E:\targetdir /NP /MIR | RXGREP "\s+\d+\s+D:\\sourcedir\\[^\n\r]*\r\n([^\n\r\\]+\r\n)+" /I
  
  will skip all directories in which no action took place and return something like:
  

                           125    D:\sourcedir\subdir\
              New File                 342        brandnewfile.ext
              Newer                  4.06m        updatedfile.ext
            *EXTRA File              2.40m        deletedfile.ext

  Hover your mouse over the regular expression or the output to highlight the "components" of the regular expression and their matches.
  
  The following regular expression will display the end results too, besides the directory and files as shown above:
  
  "((\s+\d+\s+D:\\sourcedir\\[^\r\n]*\r\n+([^\r\n\\]+\r\n+)+)|\s+Total\s+[^\r\n]+\r\n+(\s+(Dirs|Files|Bytes|Times)\s:\s+[^\r\n]+\r\n+)+)"