The type of highlighting is selected by file extension. Files with
.c, .cc, .c++, .cxx, .m, .h, .go, .js, .java, .py,
.sh, .swift extensions will be highlighted with 'C' style
highlighting. Files with .htm, .html extensions will be
highlighted as HTML. Files with .cs, .css extensions will be
highlighted as CSS. Files with .org extension will be highlighted as
Emacs Org. Files with .md extensions will be highlighted as
markdown.
The algorithm makes no attempt at parsing. The text is scanned for relevent keywords, classes, constants, strings and comments and highlighted accordingly. Therefore it will not be exactly correct, but good enough for a simple text editor. See Source Code Syntax Highlighting.
Keywords and types are matched from lists of C/C++/Objective
C/Go/Java/Javascript/Python/Shell/Swift keywords and types. Classes
are capitalised words. Constants are all caps words. Strings are in
double quotes. Single quotes are ignored because apostrophes break the
algorithm. Both /* */ and // C style comments and # Shell
script style comments are recognised.
HTML keywords are matched from a list. Double quoted arguments are
highlighted. As above, single quotes are ignored. HTML comments <!-- --> are recognised.
CSS style names are matched from a list. Double quoted arguments are highlighted. As above, single quotes are ignored. C style comments are recognised.
Metadata, headers, links, emphasis and inline code will be highlighted.
Markdown headers, links, emphasis and code will be highlighted.
Files with unrecognised extensions which are not plain text files will be highlighted with default highlighting, similar to C type highlighting. Comments will not be highlighted as there is little consistency with comment delimiters outside C type languages.
Caution Files with strings of symbols may defeat the regex processing and cause the app to stop responding.
Because scanning and highlighting a large file can be quite slow, making the app unresponsive, only the text currently in view is scanned and highlighted. Therefore as the text is edited or scrolled, the new region in view will be scanned and highlighted after a short delay to allow for user typing without the highlighting running constantly.
After the text is highlighted, the android view system will re-layout the views whether they need it or not. That causes the current cursor position to be scrolled back into view, which can be extremely annoying. So the cursor is moved if necessary to keep it within the visible region.
On devices running android versions less than Marshmallow M (6), horizontal scrolling will scroll back again. Make the text size smaller or rotate the device to avoid this. Or turn the highlighting off.