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# leex

### MODULE

leex

### MODULE SUMMARY

Lexical analyzer generator for Erlang

### DESCRIPTION

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.

### DATA TYPES

    ErrorInfo = {ErrorLine,module(),error_descriptor()}
    ErrorLine = integer()
    Token = tuple()

### EXPORTS

file(FileName) -> ok | error
file(FileName, Options) -> ok | error

Types:

FileName = filename()
Options = Option | [Option]
Option = - see below -
FileReturn = {ok, Scannerfile} | {ok, Scannerfile, Warnings} | error | {error, Warnings, Errors}
Scannerfile = filename()
Warnings = Errors = [{filename(), [ErrorInfo]}]

Generates a lexical analyzer from the definition in the input file. The input file has the extension .xrl. This is added to the filename if it is not given. The resulting module is the Xrl filename without the .xrl extension.

The current options are:

dfa_graph

  Generates a .dot file which contains a description of the DFA in a format which can be viewed with Graphviz, www.graphviz.com.

{includefile,Includefile}

  Uses a specific or customised prologue file instead of default lib/parsetools/include/leexinc.hrl which is otherwise included.

{report_errors, bool()}

  Causes errors to be printed as they occur. Default is true.

{report_warnings, bool()}

  Causes warnings to be printed as they occur. Default is true.

warnings_as_errors

  Causes warnings to be treated as errors.

{report, bool()}

  This is a short form for both report_errors and report_warnings.

{return_errors, bool()}

  If this flag is set, {error, Errors, Warnings} is returned when there are errors. Default is false.

{return_warnings, bool()}

  If this flag is set, an extra field containing Warnings is added to the tuple returned upon success. Default is false.

{return, bool()}

  This is a short form for both return_errors and return_warnings.

{scannerfile, Scannerfile}

  Scannerfile is the name of the file that will contain the Erlang scanner code that is generated. The default ("") is to add the extension .erl to FileName stripped of the .xrl extension.

{verbose, bool()}

  Outputs information from parsing the input file and generating the internal tables.

Any of the Boolean options can be set to true by stating the name of the option. For example, verbose is equivalent to {verbose, true}.

Leex will add the extension .hrl to the Includefile name and the extension .erl to the Scannerfile name, unless the extension is already there.

format_error(ErrorInfo) -> Chars

Types:

Chars = [char() | Chars]

Returns a string which describes the error ErrorInfo returned when there is an error in a regular expression.

### GENERATED SCANNER EXPORTS

The following functions are exported by the generated scanner.

### EXPORTS

string(String) -> StringRet
string(String, StartLine) -> StringRet

Types:

String = string()
StringRet = {ok,Tokens,EndLine} | ErrorInfo
Tokens = [Token]
EndLine = StartLine = integer()

Scans String and returns all the tokens in it, or an error.

Note

It is an error if not all of the characters in String are consumed.

token(Cont, Chars) -> {more,Cont1} | {done,TokenRet,RestChars}
token(Cont, Chars, StartLine) -> {more,Cont1} | {done,TokenRet,RestChars}

Types:

Cont = [] | Cont1
Cont1 = tuple()
Chars = RestChars = string() | eof
TokenRet = {ok, Token, EndLine} | {eof, EndLine} | ErrorInfo
StartLine = EndLine = integer()

This is a re-entrant call to try and scan one token from Chars. If there are enough characters in Chars to either scan a token or detect an error then this will be returned with {done,...}. Otherwise {cont,Cont} will be returned where Cont is used in the next call to token() with more characters to try an scan the token. This is continued until a token has been scanned. Cont is initially [].

It is not designed to be called directly by an application but used through the i/o system where it can typically be called in an application by:

    io:request(InFile, {get_until,Prompt,Module,token,[Line]})
      -> TokenRet

tokens(Cont, Chars) -> {more,Cont1} | {done,TokensRet,RestChars}
tokens(Cont, Chars, StartLine) -> {more,Cont1} | {done,TokensRet,RestChars}

Types:

Cont = [] | Cont1
Cont1 = tuple()
Chars = RestChars = string() | eof
TokensRet = {ok, Tokens, EndLine} | {eof, EndLine} | ErrorInfo
Tokens = [Token]
StartLine = EndLine = integer()

This is a re-entrant call to try and scan tokens from Chars. If there are enough characters in Chars to either scan tokens or detect an error then this will be returned with {done,...}. Otherwise {cont,Cont} will be returned where Cont is used in the next call to tokens() with more characters to try an scan the tokens. This is continued until all tokens have been scanned. Cont is initially [].

This functions differs from token in that it will continue to scan tokens upto and including an {end_token,Token} has been scanned (see next section). It will then return all the tokens. This is typically used for scanning grammars like Erlang where there is an explicit end token, '.'. If no end token is found then the whole file will be scanned and returned. If an error occurs then all tokens upto and including the next end token will be skipped.

It is not designed to be called directly by an application but used through the i/o system where it can typically be called in an application by:

    io:request(InFile, {get_until,Prompt,Module,tokens,[Line]})
      -> TokensRet

### 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.

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 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.