1 Lexers

1.1 Creating a Lexer

The implementation of syntax-color/racket-lexer contains a lexer for the racket language. In addition, files in the "examples" sub-directory of the "parser-tools" collection contain simpler example lexers.

An re is matched as follows:

• id — expands to the named lexer abbreviation; abbreviations are defined via define-lex-abbrev or supplied by modules like parser-tools/lex-sre.

• string — matches the sequence of characters in string.

• character — matches a literal character.

• (repetition lo hi re) — matches re repeated between lo and hi times, inclusive; hi can be +inf.0 for unbounded repetitions.

• (union re ...) — matches if any of the sub-expressions match

• (intersection re ...) — matches if all of the res match.

• (complement re) — matches anything that re does not.

• (concatenation re ...) — matches each re in succession.

• (char-range char char) — matches any character between the two (inclusive); a single character string can be used as a char.

• (char-complement re) — matches any character not matched by re. The sub-expression must be a set of characters re.

• (id datum ...) — expands the lexer macro named id; macros are defined via define-lex-trans.

Note that both (concatenation) and "" match the empty string, (union) matches nothing, (intersection) matches any string, and (char-complement (union)) matches any single character.

The regular expression language is not designed to be used directly, but rather as a basis for a user-friendly notation written with regular expression macros. For example, parser-tools/lex-sre supplies operators from Olin Shivers’s SREs, and parser-tools/lex-plt-v200 supplies (deprecated) operators from the previous version of this library. Since those libraries provide operators whose names match other Racket bindings, such as * and +, they normally must be imported using a prefix:

(require (prefix-in : parser-tools/lex-sre))

The suggested prefix is :, so that :* and :+ are imported. Of course, a prefix other than : (such as re-) will work too.

Since negation is not a common operator on regular expressions, here are a few examples, using : prefixed SRE syntax:

• (complement "1")

  Matches all strings except the string "1", including "11", "111", "0", "01", "", and so on.

• (complement (:* "1"))

  Matches all strings that are not sequences of "1", including "0", "00", "11110", "0111", "11001010" and so on.

• (:& (:: any-string "111" any-string)

  (complement (:or (:: any-string "01") (:+ "1"))))

  Matches all strings that have 3 consecutive ones, but not those that end in "01" and not those that are ones only. These include "1110", "0001000111" and "0111" but not "", "11", "11101", "111" and "11111".

• (:: "/*" (complement (:: any-string "*/" any-string)) "*/")

  Matches Java/C block comments. "/**/", "/******/", "/*////*/", "/*asg4*/" and so on. It does not match "/**/*/", "/* */ */" and so on. (:: any-string "*/" any-string) matches any string that has a "*/" in is, so (complement (:: any-string "*/" any-string)) matches any string without a "*/" in it.

• (:: "/*" (:* (complement "*/")) "*/")

  Matches any string that starts with "/*" and ends with "*/", including "/* */ */ */". (complement "*/") matches any string except "*/". This includes "*" and "/" separately. Thus (:* (complement "*/")) matches "*/" by first matching "*" and then matching "/". Any other string is matched directly by (complement "*/"). In other words, (:* (complement "xx")) = any-string. It is usually not correct to place a :* around a complement.

The following binding have special meaning inside of a lexer action:

• start-pos — a position struct for the first character matched.

• end-pos — a position struct for the character after the last character in the match.

• lexeme — the matched string.

• input-port — the input-port being processed (this is useful for matching input with multiple lexers).

• (return-without-pos x) is a function (continuation) that immediately returns the value of x from the lexer. This useful in a src-pos lexer to prevent the lexer from adding source information. For example:

  (define get-token

  (lexer-src-pos

  ...

  ((comment) (get-token input-port))

  ...))

  would wrap the source location information for the comment around the value of the recursive call. Using ((comment) (return-without-pos (get-token input-port))) will cause the value of the recursive call to be returned without wrapping position around it.

The lexer raises an exception (exn:read) if none of the regular expressions match the input. Hint: If (any-char custom-error-behavior) is the last rule, then there will always be a match, and custom-error-behavior is executed to handle the error situation as desired, only consuming the first character from the input buffer.

In addition to returning characters, input ports can return eof-objects. Custom input ports can also return a special-comment value to indicate a non-textual comment, or return another arbitrary value (a special). The non-re trigger forms handle these cases:

• The (eof) rule is matched when the input port returns an eof-object value. If no (eof) rule is present, the lexer returns the symbol 'eof when the port returns an eof-object value.

• The (special-comment) rule is matched when the input port returns a special-comment structure. If no special-comment rule is present, the lexer automatically tries to return the next token from the input port.

• The (special) rule is matched when the input port returns a value other than a character, eof-object, or special-comment structure. If no (special) rule is present, the lexer returns (void).

End-of-files, specials, special-comments and special-errors cannot be parsed via a rule using an ordinary regular expression (but dropping down and manipulating the port to handle them is possible in some situations).

Since the lexer gets its source information from the port, use port-count-lines! to enable the tracking of line and column information. Otherwise, the line and column information will return #f.

When peeking from the input port raises an exception (such as by an embedded XML editor with malformed syntax), the exception can be raised before all tokens preceding the exception have been returned.

Each time the racket code for a lexer is compiled (e.g. when a ".rkt" file containing a lexer form is loaded), the lexer generator is run. To avoid this overhead place the lexer into a module and compile the module to a ".zo" bytecode file.

1.2 Lexer Abbreviations and Macros

1.3 Lexer SRE Operators

1.4 Lexer Legacy Operators

1.5 Tokens

Each action-expr in a lexer form can produce any kind of value, but for many purposes, producing a token value is useful. Tokens are usually necessary for inter-operating with a parser generated by parser-tools/parser, but tokens not be the right choice when using lexer in other situations.

A token cannot be named error, since error it has special use in the parser.

syntax (define-empty-tokens group-id (token-id ...))

Like define-tokens, except a each token constructor token-token-id takes no arguments and returns (quote token-id).