Remove hex escape from string - ruby

Question

I have the following hex as a string: "\xfe\xff". I'd like to convert this to "feff". How do I do this?
The closest I got was "\xfe\xff".inspect.gsub("\\x", ""), which returns "\"FEFF\"".

Answer 1

"\xfe\xff".unpack("H*").first
# => "feff"

Answer 2

You are dealing with what's called an escape sequence in your double quoted string. The most common escape sequence in a double quoted string is "\n", but ruby allows you to use other escape sequences in strings too. Your string, "\xfe\xff", contains two hex escape sequences, which are of the form:
\xNN
Escape sequences represent ONE character. When ruby processes the string, it notices the "\" and converts the whole hex escape sequence to one character. After ruby processes the string, there is no \x left anywhere in the string. Therefore, looking for a \x in the string is fruitless--it doesn't exist. The same is true for the characters 'f' and 'e' found in your escape sequences: they do not exist in the string after ruby processes the string.
Note that ruby processes hex escape sequences in double quoted strings only, so the type of string--double or single quoted--is entirely relevant. In a single quoted string, the series of characters '\xfe' is four characters long because there is no such thing as a hex escape sequence in a single quoted string:
str = "\xfe"
puts str.length #=>1
str = '\xfe'
puts str.length #=>4
Regexes behave like double quoted strings, so it is possible to use an entire escape sequence in a regex:
/\xfe/
When ruby processes the regex, then just like with a double quoted string, ruby converts the hex escape sequence to a single character. That allows you to search for the single character in a string containing the same hex escape sequence:
if "abc\xfe" =~ /\xfe/
If you pretend for a minute that the character ruby converts the escape sequence "\xfe" to is the character 'z', then that if statement is equivalent to:
if "abcz" =~ /z/
It's important to realize that the regex is not searching the string for a '\' followed by an 'x' followed by an 'f' followed by an 'e'. Those characters do not exist in the string.
The inspect() method allows you to see the escape sequences in a string by nullifying the escape sequences, like this:
str = "\\xfe\\xff"
puts str
--output:--
\xfe\xff
In a double quoted string, "\\" represents a literal backslash, while an escape sequence begins with only one slash.
Once you've nullified the escape sequences, then you can match the literal characters, like the two character sequence '\x'. But it's easier to just pick out the parts you want rather than matching the parts you don't want:
str = "\xfe\xff"
str = str.inspect #=> "\"\\xFE\\xFF\""
result = ""
str.scan /x(..)/ do |groups_arr|
result << groups_arr[0]
end
puts result.downcase
--output:--
feff
Here it is with gsub:
str = "\xfe\xff"
str = str.inspect #=>"\"\\xFE\\xFF\""
str.gsub!(/
"? #An optional quote mark
\\ #A literal '\'
x #An 'x'
(..) #Any two characters, captured in group 1
"? #An optional quote mark
/xm) do
Regexp.last_match(1)
end
puts str.downcase
--output:--
feff
Remember, a regex acts like a double quoted string, so to specify a literal \ in a regex, you have to write \\. However, in a regex you don't have to worry about a " being mistaken for the end of the regex, so you don't need to escape it, like you do in a double quoted string.
Just for fun:
str = "\xfe\xff"
result = ""
str.each_byte do |int_code|
result << sprintf('%x', int_code)
end
p result
--output:--
"feff"

Answer 3

Why are you calling inspect? That's adding the extra quotes..
Also, putting that in double quotes means the \x is interpolated. Put it in single quotes and everything should be good.
'\xfe\xff'.gsub("\\x","")
=> "feff"