fansi - ANSI Control Sequence Aware String Functions
Counterparts to R string manipulation functions that account for the effects of ANSI text formatting control sequences.
Formatting Strings with Control Sequences
Many terminals will recognize special sequences of characters in strings and
change display behavior as a result. For example, on my terminal the sequence
"\033[42m"
turns text background green:
The sequence itself is not shown, but the text display changes.
This type of sequence is called an ANSI CSI SGR control sequence. Most *nix
terminals support them, and newer versions of Windows and Rstudio consoles do
too. You can check whether your display supports them by running
term_cap_test()
.
Whether the fansi
functions behave as expected depends on many factors,
including how your particular display handles Control Sequences. See ?fansi
for details, particularly if you are getting unexpected results.
Control Sequences Require Special Handling
ANSI control characters and sequences (Control Sequences hereafter) break the relationship between byte/character position in a string and display position.
For example, in "Hello \033[42mWorld, Good\033[m Night Moon!"
the space
after "World," is thirteenth displayed character, but the eighteenth actual
character ("\033" is a single character, the ESC). If we try to split the
string after the space with substr
things go wrong in several ways:
We end up cutting up our string in the middle of "World", and worse the
formatting bleeds out of our string into the prompt line. Compare to what
happens when we use substr_ctl
, the Control Sequence aware version of
substr
:
Functions
fansi
provides counterparts to the following string functions:
substr
strsplit
strtrim
strwrap
nchar
/nzchar
These are drop-in replacements that behave (almost) identically to the base counterparts, except for the Control Sequence awareness.
fansi
also includes improved versions of some of those functions, such as
substr2_ctl
which allows for width based substrings. There are also
utility functions such as strip_ctl
to remove Control Sequences and has_ctl
to detect whether strings contain them.
Most of fansi
is written in C so you should find performance of the fansi
functions to be comparable to the base functions. strwrap_ctl
is much faster,
and strsplit_ctl
is somewhat slower than the corresponding base functions.
HTML Translation
You can translate ANSI CSI SGR formatted strings into their HTML counterparts
with sgr_to_html
:
Rmarkdown
It is possible to set knitr
hooks such that R output that contains ANSI CSI
SGR is automatically converted to the HTML formatted equivalent and displayed as
intended. See the
vignette
for details.
Installation
This package is available on CRAN:
install.packages('fansi')
It has no runtime dependencies.
For the development version use:
devtools::install_github('brodieg/fansi@development')
or:
f.dl <- tempfile()
f.uz <- tempfile()
github.url <- 'https://github.com/brodieG/fansi/archive/development.zip'
download.file(github.url, f.dl)
unzip(f.dl, exdir=f.uz)
install.packages(file.path(f.uz, 'fansi-development'), repos=NULL, type='source')
unlink(c(f.dl, f.uz))
There is no guarantee that development versions are stable or even working (Travis build status: ). The master branch typically mirrors CRAN and should be stable.
Related Packages and References
- crayon, the library that started it all.
- ansistrings, which implements similar functionality.
- ECMA-48 - Control Functions For Coded Character Sets, in particular pages 10-12, and 61.
- CCITT Recommendation T.416
- ANSI Escape Code - Wikipedia for a gentler introduction.
Acknowledgments
- R Core for developing and maintaining such a wonderful language.
- CRAN maintainers, for patiently shepherding packages onto CRAN and maintaining the repository, and Uwe Ligges in particular for maintaining Winbuilder.
- Gábor Csárdi for getting me started on the journey ANSI control sequences.
- Jim Hester because covr rocks.
- Dirk Eddelbuettel and Carl Boettiger for the rocker project, and Gábor Csárdi and the R-consortium for Rhub, without which testing bugs on R-devel and other platforms would be a nightmare.
- Tomas Kalibera for rchk and the accompanying vagrant image, and rcnst to help detect errors in compiled code.
- Winston Chang for the r-debug docker container, in particular because of the valgrind level 2 instrumented version of R.
- Hadley Wickham for devtools and roxygen2.
- Yihui Xie for knitr and J.J. Allaire etal for rmarkdown, and by extension John MacFarlane for pandoc.
- Olaf Mersmann for microbenchmark, because microsecond matter.
- All open source developers out there that make their work freely available for others to use.
- Github, Travis-CI, Codecov, Vagrant, Docker, Ubuntu, Brew for providing infrastructure that greatly simplifies open source development.
- Free Software Foundation for developing the GPL license and promotion of the free software movement.