  Technical documents contain a fair amount of unnatural language, such as
tables, formulas, pseudo-codes, etc. Unnatural language can be an important
factor of confusing existing NLP tools. This paper presents an effective method
of distinguishing unnatural language from natural language, and evaluates the
impact of unnatural language detection on NLP tasks such as document
clustering. We view this problem as an information extraction task and build a
multiclass classification model identifying unnatural language components into
four categories. First, we create a new annotated corpus by collecting slides
and papers in various formats, PPT, PDF, and HTML, where unnatural language
components are annotated into four categories. We then explore features
available from plain text to build a statistical model that can handle any
format as long as it is converted into plain text. Our experiments show that
removing unnatural language components gives an absolute improvement in
document clustering up to 15%. Our corpus and tool are publicly available.
