No paste!

# I want to make sure it's just an unescaped bracket:
#
# i.e.       [
# i.e. not  \[
#
# and make sure that the backslash itself is not escaped itself, so
# this is a backslash an a bracket:
#
#          \\[
#
# Note, I have to write double \ in the grammar string itself, it's
# actually a single \.
#
grammar G {
 regex escaped   { <!after '\\'> '\\'  }
 regex unescaped { <!after <escaped> > }   # <-- nested look-behind assertion; legal?
 regex fopen     { <unescaped> '['     }
}