JavaCC: TokenManager MiniTutorial

# JavaCC [tm]: TokenManager MiniTutorial

    The JavaCC [tm] lexical specification is organized into a set of "lexical
    states".  Each lexical state is named with an identifier.  There is a
    standard lexical state called DEFAULT.  The generated token manager is
    at any moment in one of these lexical states.  When the token manager
    is initialized, it starts off in the DEFAULT state, by default.  The
    starting lexical state can also be specified as a parameter while
    constructing a token manager object.

    Each lexical state contains an ordered list of regular expressions;
    the order is derived from the order of occurrence in the input file.
    There are four kinds of regular expressions: SKIP, MORE, TOKEN, and
    SPECIAL_TOKEN.

    All regular expressions that occur as expansion units in the grammar
    are considered to be in the DEFAULT lexical state and their order of
    occurrence is determined by their position in the grammar file.

    A token is matched as follows:  All regular expressions in the current
    lexical state are considered as potential match candidates.  The
    token manager consumes the maximum number of characters from the input
    stream possible that match one of these regular expressions.  That is,
    the token manager prefers the longest possible match.  If there are
    multiple longest matches (of the same length), the regular expression
    that is matched is the one with the earliest order of occurrence in
    the grammar file.

    As mentioned above, the token manager is in exactly one state at any
    moment.  At this moment, the token manager only considers the regular
    expressions defined in this state for matching purposes. After a match,
    one can specify an action to be executed as well as a new lexical
    state to move to.  If a new lexical state is not specified, the token
    manager remains in the current state.

    The regular expression kind specifies what to do when a regular
    expression has been successfully matched:

    SKIP: Simply throw away the matched string (after executing any
          lexical action).
    MORE: Continue (to whatever the next state is) taking the matched
          string along.  This string will be a prefix of the new matched
          string.
    TOKEN: Create a token using the matched string and send it to the
           parser (or any caller).
    SPECIAL_TOKEN: Creates a special token that does not participate in
                   parsing.  Already described earlier.

    (The mechanism of accessing special tokens is at the end of this
    page)

    Whenever the end of file <EOF> is detected, it causes the creation of
    an <EOF> token (regardless of the current state of the lexical
    analyzer).  However, if an <EOF> is detected in the middle of a match
    for a regular expression, or immediately after a MORE regular
    expression has been matched, an error is reported.

    After the regular expression is matched, the lexical action is
    executed.  All the variables (and methods) declared in the
    TOKEN_MGR_DECLS region (see below) are available here for use. In
    addition, the variables and methods listed below are also available
    for use.

    Immediately after this, the token manager changes state to that
    specified (if any).

    After that the action specified by the kind of the regular expression
    is taken (SKIP, MORE, ... ). If the kind is TOKEN, the matched token
    is returned.  If the kind is SPECIAL_TOKEN, the matched token is saved
    to be returned along with the next TOKEN that is matched.

    -------------------------------------------------------------------

    The following variables are available for use within lexical actions:

    1. StringBuffer image (READ/WRITE):

    "image" (different from the "image" field of the matched token) is a
    StringBuffer variable that contains all the characters that have been
    matched since the last SKIP, TOKEN, or SPECIAL_TOKEN.  You are free
    to make whatever changes you wish to it so long as you do not assign
    it to null (since this variable is used by the generated token manager
    also).  If you make changes to "image", this change is passed on to
    subsequent matches (if the current match is a MORE).  The content of
    "image" *does not* automatically get assigned to the "image" field
    of the matched token.  If you wish this to happen, you must explicitly
    assign it in a lexical action of a TOKEN or SPECIAL_TOKEN regular
    expression.

    Example:

    <DEFAULT> MORE : { "a" : S1 }

    <S1> MORE :
    {
      "b"
        { int l = image.length()-1; image.setCharAt(l, image.charAt(l).toUpperCase()); }
        ^1                                                                             ^2
        : S2
    }

    <S2> TOKEN :
    {
      "cd" { x = image; } : DEFAULT
           ^3
    }

    In the above example, the value of "image" at the 3 points marked by
    ^1, ^2, and ^3 are:

    At ^1: "ab"
    At ^2: "aB"
    At ^3: "aBcd"

    2. int lengthOfMatch (READ ONLY):

    This is the length of the current match (is not cumulative over MORE's).
    See example below.  You should not modify this variable.

    Example:

    Using the same example as above, the values of "lengthOfMatch" are:

    At ^1: 1 (the size of "b")
    At ^2: 1 (does not change due to lexical actions)
    At ^3: 2 (the size of "cd")

    3. int curLexState (READ ONLY):

    This is the index of the current lexical state.  You should not modify
    this variable.  Integer constants whose names are those of the lexical
    state are generated into the ...Constants file, so you can refer to
    lexical states without worrying about their actual index value.

    4. inputStream (READ ONLY):

    This is an input stream of the appropriate type (one of
    ASCII_CharStream, ASCII_UCodeESC_CharStream, UCode_CharStream, or
    UCode_UCodeESC_CharStream depending on the values of options
    UNICODE_INPUT and JAVA_UNICODE_ESCAPE).  The stream is currently at
    the last character consumed for this match.  Methods of inputStream
    can be called.  For example, getEndLine and getEndColumn can be called
    to get the line and column number information for the current match.
    inputStream may not be modified.

    5. Token matchedToken (READ/WRITE):

    This variable may be used only in actions associated with TOKEN and
    SPECIAL_TOKEN regular expressions.  This is set to be the token that
    will get returned to the parser.  You may change this variable and
    thereby cause the changed token to be returned to the parser instead
    of the original one.  It is here that you can assign the value of
    variable "image" to "matchedToken.image".  Typically that's how your
    changes to "image" has effect outside the lexical actions.

    Example:

    If we modify the last regular expression specification of the
    above example to:

    <S2> TOKEN :
    {
      "cd" { matchedToken.image = image.toString(); } : DEFAULT
    }

    Then the token returned to the parser will have its ".image" field
    set to "aBcd".  If this assignment was not performed, then the
    ".image" field will remain as "abcd".

    6. void SwitchTo(int):

    Calling this method switches you to the specified lexical state.  This
    method may be called from parser actions also (in addition to being
    called from lexical actions).  However, care must be taken when using
    this method to switch states from the parser since the lexical
    analysis could be many tokens ahead of the parser in the presence of
    large lookaheads.  When you use this method within a lexical action,
    you must ensure that it is the last statement executed in the action
    (otherwise, strange things could happen).  If there is a state change
    specified using the ": state" syntax, it overrides all switchTo calls,
    hence there is no point having a switchTo call when there is an
    explicit state change specified.  In general, calling this method
    should be resorted to only when you cannot do it any other way.  Using
    this method of switching states also causes you to lose some of the
    semantic checking that JavaCC does when you use the standard syntax.

    -------------------------------------------------------------------

    Lexical actions have access to a set of class level declarations.
    These declarations are introduced within the JavaCC file using the
    following syntax:

    token_manager_decls ::=
      "TOKEN_MGR_DECLS" ":"
      "{" java_declarations_and_code "}"

    These declarations are accessible from all lexical actions.

    EXAMPLES
    --------

    Example 1: Comments

    SKIP :
    {
      "/*" : WithinComment
    }

    <WithinComment> SKIP :
    {
      "*/" : DEFAULT
    }

    <WithinComment> MORE :
    {
      <~[]>
    }

    Example 2: String Literals with actions to print the length of the
    string:

    TOKEN_MGR_DECLS :
    {
      int stringSize;
    }

    MORE :
    {
      "\"" {stringSize = 0;} : WithinString
    }

    <WithinString> TOKEN :
    {
      <STRLIT: "\""> {System.out.println("Size = " + stringSize);} : DEFAULT
    }

    <WithinString> MORE :
    {
      <~["\n","\r"]> {stringSize++;}
    }

    HOW SPECIAL TOKENS ARE SENT TO THE PARSER:

    Special tokens are like tokens, except that they are permitted to
    appear anywhere in the input file (between any two tokens).  Special
    tokens can be specified in the grammar input file using the reserved
    word "SPECIAL_TOKEN" instead of "TOKEN" as in:

    SPECIAL_TOKEN :
    {
      <SINGLE_LINE_COMMENT: "//" (~["\n","\r"])* ("\n"|"\r"|"\r\n")>
    }

    Any regular expression defined to be a SPECIAL_TOKEN may be accessed
    in a special manner from user actions in the lexical and grammar
    specifications.  This allows these tokens to be recovered during
    parsing while at the same time these tokens do not participate in the
    parsing.

    JavaCC has been bootstrapped to use this feature to automatically
    copy relevant comments from the input grammar file into the generated
    files.

    Details:

    The class Token now has an additional field:

       Token specialToken;

    This field points to the special token immediately prior to the
    current token (special or otherwise).  If the token immediately prior
    to the current token is a regular token (and not a special token),
    then this field is set to null.  The "next" fields of regular tokens
    continue to have the same meaning - i.e., they point to the next
    regular token except in the case of the EOF token where the "next"
    field is null.  The "next" field of special tokens point to the
    special token immediately following the current token.  If the token
    immediately following the current token is a regular token, the "next"
    field is set to null.

    This is clarified by the following example.  Suppose you wish to print
    all special tokens prior to the regular token "t" (but only those that
    are after the regular token before "t"):

      if (t.specialToken == null) return;
        // The above statement determines that there are no special tokens
        // and returns control to the caller.
      Token tmp_t = t.specialToken;
      while (tmp_t.specialToken != null) tmp_t = tmp_t.specialToken;
        // The above line walks back the special token chain until it
        // reaches the first special token after the previous regular
        // token.
      while (tmp_t != null) {
        System.out.println(tmp_t.image);
        tmp_t = tmp_t.next;
      }
        // The above loop now walks the special token chain in the forward
        // direction printing them in the process.