I'm aware that most modern languages use reserved words to prevent things like keywords from being used as identifiers.

Reserved words aside, let's assume a language that allows keywords to be used as identifiers. (For example, in Ruby a keyword can be used as a method name). During compilation, how would you deal with this ambiguity?

The lexing phase doesn't seem like a good fit because it would have to consider the tokens around it. The parsing phase also doesn't seem like a good fit since ideally the parser would work with tokens that are unambiguous.

If I had to design it myself, I suppose I would have the lexer yield an ambiguous token, then have another level that considers the ambiguous token in the context of the tokens around it, (e.g. does the ambiguous token follow a def keyword? Then it must be an identifier.) Then, I would hand the unambiguous tokens to the parser.

In languages that allow keywords to be used as identifiers, what is the standard way for the compiler to tell them apart?

  • 1
    It has been a while since I built a compiler in college, but I wonder what the motivation for this could be? Can't we cope with a few words in a language being reserved? In everyday use, there are not many people who are called "Honorific First Middle Last Suffix", and few entrees are named "food". Is this a problem? For whom? I guess this is why I never had children, it would be very confusing if someone was named "My Kid".
    – user251748
    Commented Oct 3, 2017 at 16:41

3 Answers 3


If you notice in Ruby, you cannot call the method named like that directly, e.g. you cannot do


You can do


Because there you can have grammar like:

*Arguments* :
    "(" ")"

*MemberExpression* :
    *MemberExpression* "." *IdentifierName*

*CallExpression* :
    *MemberExpression* *Arguments*

(Unrelated rules to the example left out for brevity)

to recognize it. It only requires separating the rule Identifier from IdentifierName:

    *IdentifierName* **but not reserved word**

    //Rules for identifier names here

If you have a starter begin like in


Then you already activated a rule like

    "begin" *indent* *statement* *outdent* "end"

And Ruby doesn't try to figure out what you mean and it will just be a block.

But for method names where a receiver appears or some other prefix it is easy to allow keywords in the grammar and e.g. Javascript does it doo.

Grammar examples taken from ecma-262

  • So in other words, the parser handles it.
    – jhewlett
    Commented Jul 24, 2013 at 5:50
  • 1
    @jhewlett: Yes, and the lexer either hands out a "IdentifierOrKeyword" token, or the parser checks if the Identifier it got matches a keyword that can appear there. Commented Jul 24, 2013 at 7:34
  • 2
    @BartvanIngenSchenau: The lexer must give out the specific token types. But the grammar accepts identifier or any of the keywords in all places where it can.
    – Jan Hudec
    Commented Jul 24, 2013 at 9:01

In .Net, each language has a different set of keywords. For example, this means that a library written in C# can use identifiers that are reserved in VB.NET. So, to use such library from VB.NET, you need some way to use keywords as identifiers.

Each language uses a different syntax to do that:

  • in C#, you prepend a @:

  • in VB.NET, you enclose it in brackets:

  • in F#, you enclose it in double backticks:

  • 3
    I was referring specifically to languages where the programmer does not need to escape the keyword, otherwise the problem is trivial.
    – jhewlett
    Commented Jul 24, 2013 at 14:02

I don't think there is a standard way.

Sometimes you will see some lexer tricks that implements rules like "pure is a keyword only if the next token is the keyword native".

At other times the grammer may employ the fact that in some circumstances all or some keywords can be interpreted as identifiers without introducing ambuguity.

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