Select-String is a cmdlet in PowerShell that allows searching of files, piped input, objects, etc for a pattern (by default a regular expression). Some examples:
PS C:\>events = get-eventlog -logname application -newest 100
PS C:\>$events | select-string -inputobject {$_.message} -pattern "failed"
This searches for the string "failed" among the last 100 events in the Application log in Event Viewer.
PS C:\>get-childitem c:\windows\system32\*.txt -recurse | select-string -pattern "Microsoft" -casesensitive
This searches all .txt files in the \system32 folder for the case-sensitive string "Microsoft".
PS C:\>get-childitem c:\ -include *.txt -recursive | select-string -pattern "password"
Similar but searches for all .txt files on the C:\ drive.
PS C:\>select-string -path process.txt -pattern idle, svchost -notmatch
This searches the process.txt file for lines that DO NOT include "idle" or "svchost".
More on Select-String at Technet.
