Lua: get a substring - regex

Question

I have for example
str = "Beamer-Template!navigation symbols#\\texttt {navigation symbols}"
print(str:gsub('[^!|#%s]+#', ''))
which prints
Beamer-Template!navigation \texttt {navigation symbols}
but it should be
Beamer-Template!\texttt {navigation symbols}
How can I catch the space?
Important is only the foo#bar. The pattern works fine for strings like
str="foo#bar!baz#foobar!nice|crazy"
-> bar!foobar!nice|crazy
but not with an additional space
str="foo#bar!baz baz#foobar!nice|crazy"
-> bar!baz foobar!nice|crazy
which should be bar!foobar!nice|crazy

Answer 1

To match makeindex entries it might be useful to use an LPEG grammar. This way you can split at the separators and even perform semantic actions, depending on the matched field.
local lpeg = assert(require"lpeg")
local C, S = lpeg.C, lpeg.S
local sep = S("#!|")
local str = C((1 - sep)^0)
local idx = str * ( "#" * str / function(match) return "#" .. match end
+ "!" * str / function(match) return "!" .. match end
+ "|" * str / function(match) return "|" .. match end)^0
print(idx:match("hello!world#foo|bar"))
$ lua test.lua
hello !world #foo |bar
Answer to the comment: Collecting the matches in a table. The matches are collected according to their prefix.
local lpeg = assert(require"lpeg")
local C, Ct, S = lpeg.C, lpeg.Ct, lpeg.S
local sep = S("#!|")
local str = C((1 - sep)^0)
local match = function(expr)
local prefix = function(prefix)
return function(match)
return prefix .. match
end
end
local idx = str * ( "#" * str / prefix("#")
+ "!" * str / prefix("!")
+ "|" * str / prefix("|"))^0
return Ct(idx):match(expr)
end
for _, str in ipairs{
"hello!world#foo|bar",
"foo#bar!baz baz#foobar!nice|crazy",
"foo#bar!baz#foobar!nice|crazy",
"Beamer-Template!navigation symbols#\\texttt {navigation symbols}"
} do
local t = match(str)
print(table.concat(t," "))
end
$ lua test.lua
hello !world #foo |bar
foo #bar !baz baz #foobar !nice |crazy
foo #bar !baz #foobar !nice |crazy
Beamer-Template !navigation symbols #\texttt {navigation symbols}