fixes #1180: only allow white spaces or \n before \documentclass{}

[^%]* may match a lot of things, including \documentclass{}

NEWS.md

@@ -32,6 +32,8 @@
 
 - the `cex` parameter was not correctly restored in the case of `opts_knit$set(global.par = TRUE)` (http://stackoverflow.com/q/35606445/559676)
 
+- for Rnw documents, when there are two instances of `\documentclass{}`, **knitr** might mistakenly treats the second instance as the the actual command to declare the document class (thanks, #1180, @ekstroem)
+
 ## MAJOR CHANGES
 
 - the default value of the package option `eval.after` is changed from `NULL` to `fig.cap`, i.e. the figure caption will always be evaluated after a code chunk is evaluated (thanks, @JoshOBrien, #1165)

R/pattern.R

@@ -11,7 +11,7 @@ all_patterns = list(
   `rnw` = list(
     chunk.begin = '^\\s*<<(.*)>>=.*$', chunk.end = '^\\s*@\\s*(%+.*|)$',
     inline.code = '\\\\Sexpr\\{([^}]+)\\}', inline.comment = '^\\s*%.*',
-    ref.chunk = '^\\s*<<(.+)>>\\s*$', header.begin = '(^|\n)[^%]*\\s*\\\\documentclass[^}]+\\}',
+    ref.chunk = '^\\s*<<(.+)>>\\s*$', header.begin = '(^|\n)\\s*\\\\documentclass[^}]+\\}',
     document.begin = '\\s*\\\\begin\\{document\\}'),
 
   `brew` = list(inline.code = '<%[=]{0,1}\\s+([^%]+)\\s+[-]*%>'),
@@ -20,7 +20,7 @@ all_patterns = list(
     chunk.begin = '^\\s*%+\\s*begin.rcode\\s*(.*)', chunk.end = '^\\s*%+\\s*end.rcode',
     chunk.code = '^%+', ref.chunk = '^%+\\s*<<(.+)>>\\s*$',
     inline.comment = '^\\s*%.*', inline.code = '\\\\rinline\\{([^}]+)\\}',
-    header.begin = '(^|\n)[^%]*\\s*\\\\documentclass[^}]+\\}',
+    header.begin = '(^|\n)\\s*\\\\documentclass[^}]+\\}',
     document.begin = '\\s*\\\\begin\\{document\\}'),
 
   `html` = list(

tests/testit/test-output.R

@@ -97,3 +97,17 @@ assert(
   'insert_header_latex() finds the correct \\documentclass{}',
   identical(res[1], '%\\documentclass{article}')
 )
+
+txt = '\\documentclass{article}
+\\begin{document}
+\\begin{verbatim}
+\\documentclass{article}
+\\end{verbatim}
+\\Sexpr{pi}
+\\end{document}'
+res = strsplit(knit(text = txt, quiet = TRUE), '\n')[[1]]
+
+assert(
+  'insert_header_latex() finds the correct \\documentclass{}',
+  identical(res[length(res) - 3], '\\documentclass{article}')
+)