On this page:
1.1 Creating a Lexer
1.2 Lexer Abbreviations and Macros
1.3 Lexer SRE Operators
1.4 Lexer Legacy Operators
1.5 Tokens

1 Lexers

 (require parser-tools/lex)

1.1 Creating a Lexer

(lexer [trigger action-expr] ...)
trigger = re
  | (eof)
  | (special)
  | (special-comment)
re = id
  | string
  | character
  | (repetition lo hi re)
  | (union re ...)
  | (intersection re ...)
  | (complement re)
  | (concatenation re ...)
  | (char-range char char)
  | (char-complement re)
  | (id datum ...)
Produces a function that takes an input-port, matches the re’s against the buffer, and returns the result of executing the corresponding action-expr.

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

An re is matched as follows:

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 Scheme 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:

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

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:

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 scheme code for a lexer is compiled (e.g. when a ".ss" 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.

(lexer-src-pos (trigger action-expr) ...)
Like lexer, but for each action-result produced by an action-expr, returns (make-position-token action-result start-pos end-pos) instead of simply action-result.

Use of these names outside of a lexer action is a syntax error.

(struct position (offset line col)
  #:extra-constructor-name make-position)
  offset : exact-positive-integer?
  line : exact-positive-integer?
  col : exact-nonnegative-integer?
Instances of position are bound to start-pos and end-pos. The offset field contains the offset of the character in the input. The line field contains the line number of the character. The col field contains the offset in the current line.

(struct position-token (token start-pos end-pos)
  #:extra-constructor-name make-position-token)
  token : any/c
  start-pos : position?
  end-pos : position?
Lexers created with src-pos-lexers return instances of position-token.

(file-path)  any/c
(file-path source)  void?
  source : any/c
A parameter that the lexer uses as the source location if it raises a exn:fail:rad error. Setting this parameter allows DrRacket, for example, to open the file containing the error.

1.2 Lexer Abbreviations and Macros

(char-set string)
A lexer macro that matches any character in string.

A lexer abbreviation that matches any character.

A lexer abbreviation that matches any string.

A lexer abbreviation that matches no string.

Lexer abbreviations that match char-alphabetic? characters, char-lower-case? characters, etc.

Defines a lexer abbreviation by associating a regular expression to be used in place of the id in other regular expression. The definition of name has the same scoping properties as a other syntactic binding (e.g., it can be exported from a module).

(define-lex-abbrevs (id re) ...)
Like define-lex-abbrev, but defines several lexer abbreviations.

(define-lex-trans id trans-expr)
Defines a lexer macro, where trans-expr produces a transformer procedure that takes one argument. When (id datum ...) appears as a regular expression, it is replaced with the result of applying the transformer to the expression.

1.3 Lexer SRE Operators

 (require parser-tools/lex-sre)

(* re ...)
Repetition of re sequence 0 or more times.
(+ re ...)
Repetition of re sequence 1 or more times.
(? re ...)
Zero or one occurrence of re sequence.
(= n re ...)
Exactly n occurrences of re sequence, where n must be a literal exact, non-negative number.
(>= n re ...)
At least n occurrences of re sequence, where n must be a literal exact, non-negative number.
(** n m re ...)
Between n and m (inclusive) occurrences of re sequence, where n must be a literal exact, non-negative number, and m must be literally either #f, +inf.0, or an exact, non-negative number; a #f value for m is the same as +inf.0.
(or re ...)
Same as (union re ...).
(: re ...)
(seq re ...)
Both forms concatenate the res.
(& re ...)
Intersects the res.
(- re ...)
The set difference of the res.
(~ re ...)
Character-set complement, which each re must match exactly one character.
(/ char-or-string ...)
Character ranges, matching characters between successive pairs of characters.

1.4 Lexer Legacy Operators

 (require parser-tools/lex-plt-v200)

The parser-tools/lex-plt-v200 module re-exports *, +, ?, and & from parser-tools/lex-sre. It also re-exports :or as :, :: as @, :~ as ^, and :/ as -.
A lexer macro that matches an empty sequence.
(~ re ...)
The same as (complement re ...).

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.

(define-tokens group-id (token-id ...))
Binds group-id to the group of tokens being defined. For each token-id, a function token-token-id is created that takes any value and puts it in a token record specific to token-id. The token value is inspected using token-id and token-value.

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

(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).

(token-name t)  symbol?
  t : (or/c token? symbol?)
Returns the name of a token that is represented either by a symbol or a token structure.

(token-value t)  any/c
  t : (or/c token? symbol?)
Returns the value of a token that is represented either by a symbol or a token structure, returning #f for a symbol token.

(token? v)  boolean?
  v : any/c
Returns #t if val is a token structure, #f otherwise.