Lexical analyzer generator for Erlang

A regular expression based lexical analyzer generator for Erlang, similar to lex or flex.

Note

The Leex module should be considered experimental as it will be subject to changes in future releases.

Default Leex Options

The (host operating system) environment variable ERL_COMPILER_OPTIONS can be used to give default Leex options. Its value must be a valid Erlang term. If the value is a list, it is used as is. If it is not a list, it is put into a list.

The list is appended to any options given to file/2.

The list can be retrieved with compile:env_compiler_options/0.

Input File Format

Erlang style comments starting with a % are allowed in scanner files. A definition file has the following format:

<Header>

Definitions.

<Macro Definitions>

Rules.

<Token Rules>

Erlang code.

<Erlang code>

The "Definitions.", "Rules." and "Erlang code." headings are mandatory and must occur at the beginning of a source line. The <Header>, <Macro Definitions> and <Erlang code> sections may be empty but there must be at least one rule.

Macro definitions have the following format:

NAME = VALUE

and there must be spaces around =. Macros can be used in the regular expressions of rules by writing {NAME}.

Note

When macros are expanded in expressions the macro calls are replaced by the macro value without any form of quoting or enclosing in parentheses.

Rules have the following format:

<Regexp> : <Erlang code>.

The <Regexp> must occur at the start of a line and not include any blanks; use \t and \s to include TAB and SPACE characters in the regular expression. If <Regexp> matches then the corresponding <Erlang code> is evaluated to generate a token. With the Erlang code the following predefined variables are available:

• TokenChars - A list of the characters in the matched token.

• TokenLen - The number of characters in the matched token.

• TokenLine - The line number where the token occurred.

• TokenCol - The column number where the token occurred (column of the first character included in the token).

• TokenLoc - Token location. Expands to {TokenLine,TokenCol} (even when error_location is set to line.

The code must return:

• {token,Token} - Return Token to the caller.

• {end_token,Token} - Return Token and is last token in a tokens call.

• skip_token - Skip this token completely.

• {error,ErrString} - An error in the token, ErrString is a string describing the error.

It is also possible to push back characters into the input characters with the following returns:

• {token,Token,PushBackList}
• {end_token,Token,PushBackList}
• {skip_token,PushBackList}

These have the same meanings as the normal returns but the characters in PushBackList will be prepended to the input characters and scanned for the next token. Note that pushing back a newline will mean the line numbering will no longer be correct.

Note

Pushing back characters gives you unexpected possibilities to cause the scanner to loop!

The following example would match a simple Erlang integer or float and return a token which could be sent to the Erlang parser:

D = [0-9]

{D}+ :
  {token,{integer,TokenLine,list_to_integer(TokenChars)}}.

{D}+\.{D}+((E|e)(\+|\-)?{D}+)? :
  {token,{float,TokenLine,list_to_float(TokenChars)}}.

The Erlang code in the "Erlang code." section is written into the output file directly after the module declaration and predefined exports declaration so it is possible to add extra exports, define imports and other attributes which are then visible in the whole file.

Regular Expressions

The regular expressions allowed here is a subset of the set found in egrep and in the AWK programming language, as defined in the book, The AWK Programming Language, by A. V. Aho, B. W. Kernighan, P. J. Weinberger. They are composed of the following characters:

• c - Matches the non-metacharacter c.

• \c - Matches the escape sequence or literal character c.

• . - Matches any character.

• ^ - Matches the beginning of a string.

• $ - Matches the end of a string.

• [abc...] - Character class, which matches any of the characters abc.... Character ranges are specified by a pair of characters separated by a -.

• [^abc...] - Negated character class, which matches any character except abc....

• r1 | r2 - Alternation. It matches either r1 or r2.

• r1r2 - Concatenation. It matches r1 and then r2.

• r+ - Matches one or more rs.

• r* - Matches zero or more rs.

• r? - Matches zero or one rs.

• (r) - Grouping. It matches r.

The escape sequences allowed are the same as for Erlang strings:

• \b - Backspace.

• \f - Form feed.

• \n - Newline (line feed).

• \r - Carriage return.

• \t - Tab.

• \e - Escape.

• \v - Vertical tab.

• \s - Space.

• \d - Delete.

• \ddd - The octal value ddd.

• \xhh - The hexadecimal value hh.

• \x{h...} - The hexadecimal value h....

• \c - Any other character literally, for example \\ for backslash, \" for ".

The following examples define simplified versions of a few Erlang data types:

Atoms [a-z][0-9a-zA-Z_]*

Variables [A-Z_][0-9a-zA-Z_]*

Floats (\+|-)?[0-9]+\.[0-9]+((E|e)(\+|-)?[0-9]+)?

Note

Anchoring a regular expression with ^ and $ is not implemented in the current version of Leex and just generates a parse error.