How can I include quotes for string and characters as part of the string. Example is "This is a \" string" which should result in one string instead of "This is a \" as one string and string" as an error in this case. The same goes for the characters. Example is '\'', but
in my case it's only '\'.
This is my current solution which works only without quotes.
CHARACTER
: '\'' ~('\'')+ '\''
;
STRING
: '"' ~('"')+ '"'
;
Your string/char rules don't handle escape sequences correctly. For the character it should be:
CHARACTER: '\'' '\\'? '.' '\'';
Here we make the escape char (backshlash) be part of the rule and require an additional char (whatever it is) follow it. Similar for the string:
STRING: '"' ('\\'? .)+? '"';
By using +? we are telling ANTLR4 to match in a non-greedy manner, stopping at the first non-escaped quote char after the initial one.
Related
I would like to use a string which contains the character in ocaml '"', for example the string
"This character '"' is interesting".
I've written "This character '\"' is interesting". But it doesn't work...
Quoting it like you did is working, but you could also use the new string delimiter:
let () =
print_endline "This character '\"' is interesting";
print_endline {|This character '"' is interesting|}
The requirement for the assignment is:
"Illegal escape in string: " + wrong string: When the lexer detects an illegal
escape in string. The wrong string is from the beginning of the string to the
illegal escape.
All the supported escape sequences are as follows:
\b backspace
\f formfeed
\r carriage return
\n newline
\t horizontal tab
\’ single quote
\" double quote
\ backslash
I use the code for "String" as same as this post recommended:
ANTLR4 - Need an explanation on this String Literals
STRINGLIT: '"' ( '\\' [btnfr"'\\] | ~[\b\t\f\r\n\\"] )* '"';
And also fix a little bit for "Unterminated (or Unclosed) String" as follow:
UNCLOSE_STRING: '"' ( '\\' [btnfr"'\\] | ~[\b\t\f\r\n\\"] )* ;
So I tried to write down the prototype for that requirement like this:
ILLEGAL_ESCAPE: '"' .*? ESCAPE ;
fragment ESCAPE: [\b\f\r\n\t'"\\]
Can someone help me to figure out if had done something wrong to it, I think there is something not clear between STRING and ILLEGAL_ESCAPE so the result is not right.
I appreciate if you can fix it again to meet the requirement as I mentioned earlier. Thanks in advance!!
Try to use the following lexer rule:
ILLEGAL_ESCAPE: '"' ('\\' ~[btnfr"'\\] | ~'\\')*;
I'm trying to implement a parser using ANTLRv4 for a language that accepts both "" and \" as a way escaping " characters in " delimited strings.
The answers to this question show how to do it for "" escaping. However when I try to extend it to also cover the \" case, it almost works but becomes too greedy when two strings are on the same line.
Here is my grammar:
grammar strings;
strings : STRING (',' STRING )* ;
STRING
: '"' (~[\r\n"] | '""' | '\"' )* '"'
;
Here is my input of three strings:
"This is ""my string\"",
"cat","fish"
This correctly recognises "This is ""my string\"", but thinks that "cat","fish" is all one string.
If I move "fish" down on to the next line it works correctly.
Can anyone figure out how to make it work if "cat" and "fish" are on the same line?
Make your STRING rule non greedy to stop at the first quote char it encounters, instead of trying to get as much as possible:
STRING
: '"' (~[\r\n"] | '""' | '\"' )*? '"'
;
I've found what I need to do to get this to work as I wanted, though to be honest I'm still not entirely sure why Antlr was doing what it did.
Simply by adding another backslash character to the '\"' clause it works!
So my final STRINGS definition is : '"' (~[\r\n"] | '""' | '\\"' )* '"'
Going back to first principles, I hand drew a state transition diagram of the problem and then realised that the two escaping mechanism sequences are not the same and cannot be treated similarly. Then trying to implement the two patterns in AntlrWorks it became apparent that I needed to add the second backslash at which point it all started working.
Does a single backslash followed by some arbitrary character simply mean that character?
How do I write a lexer rule to match a String literal which does not end in an escaped quote?
Here's my grammar:
lexer grammar StringLexer;
// from The Definitive ANTLR 4 Reference
STRING: '"' (ESC|.)*? '"';
fragment ESC : '\\"' | '\\\\' ;
Here's my java block:
String s = "\"\\\""; // looks like "\"
StringLexer lexer = new StringLexer(new ANTLRInputStream(s));
Token t = lexer.nextToken();
if (t.getType() == StringLexer.STRING) {
System.out.println("Saw a String");
}
else {
System.out.println("Nope");
}
This outputs Saw a String. Should "\" really match STRING?
Edit: Both 280Z28 and Bart's solutions are great solutions, unfortunately I can only accept one.
For properly formed input, the lexer will match the text you expect. However, the use of the non-greedy operator will not prevent it from matching something with the following form:
'"' .*? '"'
To ensure strings are tokens in the most "sane" way possible, I recommended using the following rules.
StringLiteral
: UnterminatedStringLiteral '"'
;
UnterminatedStringLiteral
: '"' (~["\\\r\n] | '\\' (. | EOF))*
;
If your language allows string literals to span across multiple lines, you would likely need to modify UnterminatedStringLiteral to allow matching end-of-line characters.
If you do not include the UnterminatedStringLiteral rule, the lexer will handle unterminated strings by simply ignoring the opening " character of the string and proceeding to tokenize the content of the string.
Yes, "\" is matched by the STRING rule:
STRING: '"' (ESC|.)*? '"';
^ ^ ^
| | |
// matches: " \ "
If you don't want the . to match the backslash (and quote), do something like this:
STRING: '"' ( ESC | ~[\\"] )* '"';
And if your string can't be spread over multiple lines, do:
STRING: '"' ( ESC | ~[\\"\r\n] )* '"';
I have an assignment to create a lexical analyser and I've got everything working except for one bit.
I need to create a string that will accept a new line, and the string is delimited by double quotes.
The string accepts any number, letter, some specified punctuation, backslashes and double quotes within the delimiters.
I can't seem to figure out how to escape a new line character.
Is there a certain way of escaping characters like new line and tab?
Here's some of my code that might help
< STRING : ( < QUOTE> (< QUOTE > | < BACKSLASH > | < ID > | < NUM > | " " )* <QUOTE>) >
< #QUOTE : "\"" >
< #BACKSLASH : "\\" >
So my string should allow for a quote, then any of the following characters like a backslash, a whitespace, a number etc, and then followed by another quote.
The newline char like "\n" is what's not working.
Thanks in advance!
For string literals, JavaCC borrows the syntax of Java. So, a single-character literal comprising a carriage return is escaped as "\r", and a single-character literal comprising a line feed is escaped as "\n".
However, the processed string value is just a single character; it is not the escape itself. So, suppose you define a token for line feed:
< LF : "\n" >
A match of the token <LF> will be a single line-feed character. When substituting the token in the definition of another token, the single character is effectively substituted. So, suppose you have the higher-level definition:
< STRING : "\"" ( <LF> ) "\"" >
A match of the token <STRING> will be three characters: a quotation mark, followed by a line feed, followed by a quotation mark. What you seem to want instead is for the escape sequence to be recognized:
< STRING : "\"" ( "\\n" ) "\"" >
Now a match of the token <STRING> will be four characters: a quotation mark, followed by an escape sequence representing a line feed, followed by a quotation mark.
In your current definition, I see that other often-escaped metacharacters like quotation mark and backslash are also being recognized literally, rather than as escape sequences.