Convert a string with '\b' escape character (backspace) to a normal string

Question

My string:

std::string With_esc = "asd\b";

I want to convert it to a simple string "as" (apply the backspace character and forget it). Is there any way to do this in C++? It should look like this:

std::string With_esc = "asd\b";
std::string Without_esc = With_esc; //Here I should convert it
std::ofstream FWith_esc ("with");
std::ofstream FWithout_esc ("without");
FWithout_esc << Without_esc;
FWith_esc << With_esc;

Bash:

~ cat -e with
  asd^H
~ cat -e without
  as

Unfortunately I don't know how to convert it, so both files look exactly the same.

best chance is to do a search'n'replace yourself, it shouldn't be too hard — Marco A., Jul 30 '14 at 13:19
@MarcoA. Well replacing the '\b' character with '' will not consume it. The string variable will still contain "asd" instead of "as". — Tomasz Kasperczyk, Jul 30 '14 at 13:22
@AdrianoRepetti Would you want `"aa\b\b"` to become `"a\b"`, or do you want it to become `""`? — , Jul 30 '14 at 13:23
@user3125731 if the backspace character is the only one you need to deal with you can take care of the previous digit as well. If not then it might be a problem (or a lot of work). There might be a better solution I don't see right now (except passing stuff in a bash shell) — Marco A., Jul 30 '14 at 13:23
And what should happen if the `'\b'` is the first character of the string? — James Kanze, Jul 30 '14 at 13:24
@hvd good question...I assumed just one escape otherwise he may need looping (or a better regex, I'm really not such good with them) — Adriano Repetti, Jul 30 '14 at 13:24

score 4 · Accepted Answer · edited May 23 '17 at 11:55

4

Assuming you're lucky enough to use C++11 (otherwise adapt this code to your favorite regex engine):

string With_esc = R"asd\b";
string Without_esc = regex_replace(With_esc, regex(".\\b"), "");

As pointed out in comments this approach has following limitations:

It'll replace only one "back" so if you have "12\b\b" you'll get "1\b". To handle this you need to loop until input and output (for regex_replace()) are different (or maybe a better regex, I'm not such good with them).
It won't handle \b as beginning of the string like in "\b123". To handle this you need a simple string replacement (using technique suggested by Giobunny) to remove \b after a regex_replace().
Note here I'm using verbatim strings (it's ok for an example if your text comes from a file but if you're working with string literals you need to update regex accordingly).

UPDATE
As noted by Eric Finn this expression will also match multiple backspaces then "a\b\b\b" will become "\b\b" and then "\", that's obviously wrong. As he suggested a better regex should include check for "[^\b]\b" too.

edited May 23 '17 at 11:55

Community

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answered Jul 30 '14 at 13:22

Adriano Repetti

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Simplest way to go IMHO. +1 – Marco A. Jul 30 '14 at 13:24
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Why do you need the parentheses in the regular expression? – James Kanze Jul 30 '14 at 13:25
Fortunately I'm using C++11. What is reg() function? I can't find it anywhere. – Tomasz Kasperczyk Jul 30 '14 at 13:28
You can use C++11 [raw string literals](http://en.cppreference.com/w/cpp/language/string_literal) for regex expressions for less hassle with escape chars, e.g. `std::regex{R"(.\b)"}` (parentheses needed for raw string literal). – Felix Glas Jul 30 '14 at 13:30
Strange, because with updated gcc (4.9.1) and std=c++11 flag I get ‘reg’ was not declared in this scope error. – Tomasz Kasperczyk Jul 30 '14 at 13:34
@user3125731 just include "regex" header (and std namespace). – Adriano Repetti Jul 30 '14 at 13:34
@AdrianoRepetti that's exactly what I did but it didn't solve the problem. – Tomasz Kasperczyk Jul 30 '14 at 13:37
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@user3125731 ROFL sorry, typo: it's not "reg" but "regex"! – Adriano Repetti Jul 30 '14 at 13:38
Shouldn't this be something like `[^\b]\b`? I would think that this regex would match `a\b\b\b` in `aaa\b\b\b`. – Eric Finn Jul 30 '14 at 13:52
@EricFinn yes but (maybe) OP wants to transform "aaa\b\b\b" in "" (emtpy string: 3 "a" deleted by 3 backspaces). – Adriano Repetti Jul 30 '14 at 14:03
@AdrianoRepetti Exactly. However, if you match `a\b\b\b`, you will have a backspace deleting an `a` and a backspace deleting a backspace. – Eric Finn Jul 30 '14 at 14:51

score 2 · Answer 2 · answered Aug 01 '14 at 07:36

Try something like this, if you don't want to use a regex:

std::string convertBackspaces(std:string str)
{
    std::string::iterator iter = str.begin();
    std::string::iterator end = str.end();
    while (iter != end)
    {
        iter = std::find(iter, end, '\b');
        if (iter == end) break;
        if (iter == str.begin())
            iter = str.erase(iter);
        else
            iter = str.erase(iter-1, iter+1);
        end = str.end();
    }
    return str;
}

std::string Without_esc = convertBackspaces(With_esc);

score 0 · Answer 3 · edited May 23 '17 at 10:33

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You can replace the substring "\b" with an empty string. It's not the fastest/safest method but it will work, you can follow.

Replace substring with another substring C++

edited May 23 '17 at 10:33

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answered Jul 30 '14 at 13:21

Giobunny

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He wants it to also remove the previous character; \b is backspace – RJFalconer Jul 30 '14 at 13:30

Convert a string with '\b' escape character (backspace) to a normal string

3 Answers3