Sed it the streaming editor. It makes it easy for example to replace specially marked texts inside a source code file. The Wireshark build process uses this to stamp version strings into various places.
Sed is available for most of the UNIX-like platforms and as the sed package from the Cygwin setup.
If sed isn't already installed or available as a package for your platform, you can get it at: http://directory.fsf.org/GNU/sed.html
After correct installation, typing at the bash command line prompt:
$
sed --version
should result in something like:
GNU sed version 4.1.5
Copyright (C) 2003 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE,
to the extent permitted by law.
However, the version string may vary.
A native Win32 sed package can be obtained from http://gnuwin32.sourceforge.net/. The installation should be straightforward.