Lex/Flex :Regular expression for string literals in C/C++?

I look here [ANSI C grammar][1] .


[1]:

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This page includes a lot of regular expressions in Lex/Flex for ANSI C.

Having a problem in understanding regular expression for string literals.

They have mentioned regular expression as `\"(\\.|[^\\"])*\"`

As I can understand `\"` this is used for double quotes, `\\` is for escape character, `.` is for any character except `escape character` and `*` is for zero or more times.

`[^\\"]` implies characters except `\` , `"` .

So, in my opinion, regular expression should be `\"(\\.)*\"`.

Can you give some strings where above regular expression will fail?

or

Why they have used `[^\\"]`?

The regex `\"(\\.)*\"` that you proposed matches strings that consist of `\` symbols alternating with any characters like:

"\z\x\p\r"

This regular expression would therefore fail to match a string like:

"hello"

The string `"hello"` would be matched by the regex `\".*\"` but that would also match the string `""""` or `"\"` both of which are invalid.

To get rid of these invalid matches we can use `\"[^\\"]*\"`, but this will now fail to match a string like `"\a\a\a"` which is a valid string.

As we saw `\"(\\.)*\"` does match this string, so all we need to do is combine these two to get `\"(\\.|[^\\"])*\"`.