GPP - GPP is a general-purpose preprocessor with customizable syntax, suitable for a wide range of...
XMLStarlet - XMLStarlet Command Line XML Toolkit
Gema - General purpose text macro processor.
xmllint - command line XML tool
GCC C Preprocessor (cpp) - Top (The C Preprocessor)
PP - A generic Preprocessor - P is a text preprocessor designed for Pandoc (and more generally Markdown and reStructuredText).