Replacing strings in lua containing special characters

Question

I want to replace string in lua. Here is the string.

strng='\begin{matrix}   1 & 2 & 3 \\    4 & 5 & 6 \\    7 & 8 & 10 \end{matrix}'

I want to replace

\begin{matrix} by {{
& by ,
\\ by },{
\end{matrix} by }}

I also want to remove all spaces. So the output will be

{{1,2,3},{4,5,6},{7,8,10}}

I have written the following function to do this.

function tempsubst(m1)
m1 = matrixprint(m1)
if type(m1) ~="string" then  return  m1 end
m1 = string.gsub(m1,"\begin%{matrix%}","{{" )
m1 = string.gsub(m1,"\\","},{" )
m1 = string.gsub(m1,"%&","," )
m1 = string.gsub(m1,"end%{matrix%}","}}" )
m1= string.gsub(m1 , "%s+", "")
return m1
end

This sometimes work but sometimes it does not work. There must be mistakes in the function. I am newbie to lua. Could the code be corrected? Any help will be appreciated. Thanks.

sometimes it does not work is not the best problem description. provide example inputs and outputs. — Piglet, Jul 26 '19 at 07:44
If input is passed from some other function, it gives unexpected output. tempsubst(strng) works but tempsubst(somefunction(x)) doesn't work. Note that somefunction(x) gives string as output. I think apostrophe may be causing the problem. — Maths89, Jul 26 '19 at 07:52

score 1 · Answer 1 · answered Jul 26 '19 at 11:32

1

When confused with escapes and backslashes, it's better to use long strings, which don't interpret those:

m1=[[\begin{matrix}   1 & 2 & 3 \\    4 & 5 & 6 \\    7 & 8 & 10 \end{matrix}]]
print(m1)
m1 = string.gsub(m1,[[\begin{matrix}]],"{{" )
m1 = string.gsub(m1,[[\\]],"},{" )
m1 = string.gsub(m1,[[&]],"," )
m1 = string.gsub(m1,[[\end{matrix}]],"}}" )
m1= string.gsub(m1 , [[%s+]], "")
print(m1)

answered Jul 26 '19 at 11:32

lhf

70,581
9
108
149

Can you tell me how to convert m1='\begin{matrix} 1 & 2 & 3 \\ 4 & 5 & 6 \\ 7 & 8 & 10 \end{matrix}' to m1=[[\begin{matrix} 1 & 2 & 3 \\ 4 & 5 & 6 \\ 7 & 8 & 10 \end{matrix}]]? – Maths89 Jul 26 '19 at 14:29

Piglet · Answer 2 · 2019-07-26T08:54:21.197

0

First of all your strings contain two escape sequences "\b" and "\e".

I suppose you mean "\\b"? Escape each backslash!

Then you have the problem that you replace "\\" at any position with "},{". But at "\\end" you don't want that.

Because of that, after fixing the escape problem you get this output:

{{1,2,3},{4,5,6},{7,8,10},{}}

So either replace \\ends first or change the other pattern so it won't affect "\\end"

Edit: if you get a string from outside like

\\

your pattern is "\\\\" as you have to create a literal string with 2 backslashs which requires you to escape both, resulting in four backslashs. This pattern matches two backslash characters in a string.

edited Jul 26 '19 at 08:54

answered Jul 26 '19 at 08:17

Piglet

27,501
3
20
43

I am still not able to get. Hope I will understand. – Maths89 Jul 26 '19 at 08:29
@Maths89 what is it you don't get? – Piglet Jul 26 '19 at 08:39
I can't have string with \\b as it is output from some other function. I don't want ready made code but as code length is not too long, could you provide working solution? – Maths89 Jul 26 '19 at 08:41
@Maths89 well I just used your example. if the string comes form outside you still lack backslashs in your patterns. – Piglet Jul 26 '19 at 08:50
So there is no solution to the problem. – Maths89 Jul 26 '19 at 08:56
@Maths89 of course there is a solution. did you fix the patterns? – Piglet Jul 26 '19 at 09:00
@Maths89 Are you trying to state that the function returns the string and it is improperly escaped? or at least inconsistently escaped, sometimes 2 \ sometimes \ ? – Nifim Jul 26 '19 at 17:44

Replacing strings in lua containing special characters

2 Answers2

Linked