\starttext
\ctxlua{context(string.format("\%.2f", math.pi))}
\stoptext

mtx-context     | warning: synctex is enabled
mtx-context     | run 1: luatex --fmt="/usr/local/texlive/2015/texmf-var/luatex-cache/context/a86c089b384a3076dc514ba966a1fac9/formats/luatex/cont-en" --jobname="luapercent" --lua="/usr/local/texlive/2015/texmf-var/luatex-cache/context/a86c089b384a3076dc514ba966a1fac9/formats/luatex/cont-en.lui" --no-parse-first-line --synctex=1 --c:autogenerate --c:currentrun=1 --c:directives="system.synctex=1" --c:fulljobname="/Users/acailliau/Desktop/luapercent.tex" --c:input="/Users/acailliau/Desktop/luapercent.tex" --c:kindofrun=1 --c:maxnofruns=9 --c:synctex="1" "cont-yes.mkiv"
This is LuaTeX, Version beta-0.80.0 (TeX Live 2015) (rev 5238) 
 \write18 enabled.
system          > synctex functionality is enabled (1)!
open source     > 1 > 1 > /usr/local/texlive/2015/texmf-dist/tex/context/base/cont-yes.mkiv
ConTeXt  ver: 2015.05.18 12:26 MKIV current  fmt: 2016.4.25  int: english/english
system          > 'cont-new.mkiv' loaded
open source     > 2 > 2 > /usr/local/texlive/2015/texmf-dist/tex/context/base/cont-new.mkiv
close source    > 2 > 2 > /usr/local/texlive/2015/texmf-dist/tex/context/base/cont-new.mkiv
system          > files > jobname 'luapercent', input '/Users/acailliau/Desktop/luapercent', result 'luapercent'
fonts           > latin modern fonts are not preloaded
languages       > language 'en' is active
open source     > 2 > 3 > /Users/acailliau/Desktop/luapercent.tex
fonts           > preloading latin modern fonts (second stage)
fonts           > 'fallback modern-designsize rm 12pt' is loaded
lua error       > error on line 3 in file /Users/acailliau/Desktop/luapercent.tex:
[\directlua]:1: invalid escape sequence near '\%'
1     \starttext
2     
3 >>  \ctxlua{context(string.format("\%.2f", math.pi))}
4     
5     \stoptext

? Type  to proceed, S to scroll future error messages,
R to run without stopping, Q to run quietly,
I to insert something, 
1 or ... or 9 to ignore the next 1 to 9 tokens of input,
H for help, X to quit.
? close source    > 2 > 3 > /Users/acailliau/Desktop/luapercent.tex
close source    > 1 > 3 > /usr/local/texlive/2015/texmf-dist/tex/context/base/cont-yes.mkiv
mkiv lua stats  > used config file: selfautoparent:/texmfcnf.lua;selfautoparent:/texmf-dist/web2c/texmfcnf.lua
mkiv lua stats  > used cache path: /usr/local/texlive/2015/texmf-var/luatex-cache/context/a86c089b384a3076dc514ba966a1fac9
mkiv lua stats  > synctex tracing: synctex has been enabled (extra log file generated)
mkiv lua stats  > resource resolver: loadtime 0.117 seconds, 0 scans with scantime 0.000 seconds, 0 shared scans, 8 found files, scanned paths: 
mkiv lua stats  > stored bytecode data: 371 modules (0.309 sec), 78 tables (0.008 sec), 449 chunks (0.316 sec)
mkiv lua stats  > traced context: maxstack: 1132, freed: 0, unreachable: 1132
mkiv lua stats  > running in nuts mode: yes
mkiv lua stats  > cleaned up reserved nodes: 41 nodes, 439 lists of 438
mkiv lua stats  > node memory usage: 2 glue, 2 penalty, 6 attribute, 26 glue_spec, 2 attribute_list, 2 temp
mkiv lua stats  > node list callback tasks: 6 unique task lists, 2 instances (re)created, 3 calls
mkiv lua stats  > used backend: pdf (backend for directly generating pdf output)
mkiv lua stats  > loaded patterns: en::2, load time: 0.000
mkiv lua stats  > jobdata time: 0.001 seconds saving, 0.000 seconds loading
mkiv lua stats  > callbacks: 60 direct, 243 indirect, 303 total
mkiv lua stats  > result saved in file: luapercent.pdf, compresslevel 3, objectcompresslevel 3
mkiv lua stats  > loaded fonts: 2 files: latinmodern-math.otf, lmroman12-regular.otf
mkiv lua stats  > fonts load time: 0.180 seconds 
mkiv lua stats  > used platform: osx-64, type: unix, binary subtree: bin
mkiv lua stats  > luatex banner: this is luatex, version beta-0.80.0 (tex live 2015) (rev 5238)
mkiv lua stats  > control sequences: 43101 of 65536 + 100000
mkiv lua stats  > lua properties: engine: lua, used memory: 67 MB (ctx: 66 MB), hash type: lua, hash chars: min(64,40), symbol mask: utf (τεχ)
mkiv lua stats  > runtime: 0.540 seconds
mtx-context     | fatal error: return code: 1