In ActionScript, if you want to restrict the characters that can be entered into a TextField (or TextInput), you use the restrict property. Check here for documentation.
For example, input_txt.restrict = "A-Z";
This restricts the characters to upperspace characters and disallows everything else.
And input_txt.restrict = "^0-9";
The ^ symbol indicates that whatever comes after that is disallowed. Here, the numbers 0 to 9 are not allowed, everything else is allowed.
The interesting part is what if you want to restrict the Backslash(\) character?
Apparently, the right way to do it is:
input_txt.restrict = "^\\\\";
From the livedocs documentation:
"You can use a backslash to enter a ^ or - verbatim. The accepted backslash sequences are \-, \^ or \\. The backslash must be an actual character in the string, so when specified in ActionScript, a double backslash must be used. "
So, the first two backslashes are as required, when you add the third backslash, the flex compiler thinks that you are trying to escape the " character. Hence you need the fourth backslash in the string.