leex — parsetools v2.6 (2024)

Lexical analyzer generator for Erlang

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

Note

The leex module was considered experimental when it was introduced.

Default Leex Options

The (host operating system) environment variable ERL_COMPILER_OPTIONS can beused to give default Leex options. Its value must be a valid Erlang term. If thevalue 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. Adefinition file has the following format:

<Header>Definitions.<Macro Definitions>Rules.<Token Rules>Erlang code.<Erlang code>

The Definitions., Rules., and Erlang code headings are mandatoryand must start at the beginning of a source line. The <Header>,<Macro Definitions>, and <Erlang code> sections are allowed to beempty, 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 regularexpressions of rules by writing {NAME}.

Note

When macros are expanded in expressions, the macro calls are replaced by themacro 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 atoken. 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 thefirst character included in the token).

• TokenLoc - Token location. Expands to {TokenLine,TokenCol} (even whenerror_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 stringdescribing the error.

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

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

These have the same meanings as the normal returns but the characters inPushBackList will be prepended to the input characters and scanned for thenext token. Note that pushing back a newline will mean the line numbering willno longer be correct.

Note

Pushing back characters gives you unexpected possibilities to cause thescanner to loop!

The following example would match a simple Erlang integer or float and return atoken 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 filedirectly after the module declaration and predefined exports declaration, makingit possible to add extra exports, define imports, and other attributes, which arevisible in the whole file.

Regular Expressions

The regular expressions allowed here is a subset of the set found in egrep andin the AWK programming language, as defined in the book The AWK ProgrammingLanguage by A. V. Aho, B. W. Kernighan, and P. J. Weinberger. They are composed ofthe 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 charactersabc.... Character ranges are specified by a pair of characters separated bya -.

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

• 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.

  See Also

  Steering your submarine with Elixir, Leex and Yecc (AoC'21, day 2)Tokenizing and parsing in Elixir with yecc and leexErlang - Leex and Yecc

• \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 thecurrent version of leex and generates a parse error.