The README File for SimpleExamples
This directory contains five examples to get you started using JavaCC. Each example is contained in a single grammar file and are listed below: Simple1.jj Simple2.jj Simple3.jj Simple4.jj NL_Xlator.jj Once you have tried out and understood each of these examples, you should take a look at more complex examples in other sub-directories under the examples directory. But even with just these examples, you should be able to get started on reasonable complex grammars. --------------------------------------------------------------------- Summary Instructions: If you are a parser and lexical analyzer expert and can understand the examples by just reading them, the following instructions show you how to get started with javacc. The instructions below are with respect to Simple1.jj, but you can build any parser using the same set of commands. 1. Run javacc on the grammar input file to generate a bunch of Java files that implement the parser and lexical analyzer (or token manager): javacc Simple1.jj 2. Now compile the resulting Java programs: javac *.java 3. The parser is now ready to use. To run the parser, type: java Simple1 The Simple1 parser and others in this directory are designed to take input from standard input. Simple1 recognizes matching braces followed by zero or more line terminators and then an end of file. Examples of legal strings in this grammar are: "{}", "{{{{{}}}}}", etc. Examples of illegal strings are: "{{{{", "{}{}", "{}}", "{{}{}}", "{ }", "{x}", etc. Try typing various different inputs to Simple1. Remembermay be used to indicate the end of file (this is on the UNIX platform). Here are some sample runs: % java Simple1 {{}} % % java Simple1 {x Lexical error at line 1, column 2. Encountered: "x" TokenMgrError: Lexical error at line 1, column 2. Encountered: "x" (120), after : "" at Simple1TokenManager.getNextToken(Simple1TokenManager.java:146) at Simple1.getToken(Simple1.java:140) at Simple1.MatchedBraces(Simple1.java:51) at Simple1.Input(Simple1.java:10) at Simple1.main(Simple1.java:6) % % java Simple1 {}} ParseException: Encountered "}" at line 1, column 3. Was expecting one of: "\n" ... "\r" ... at Simple1.generateParseException(Simple1.java:184) at Simple1.jj_consume_token(Simple1.java:126) at Simple1.Input(Simple1.java:32) at Simple1.main(Simple1.java:6) % --------------------------------------------------------------------- DETAILED DESCRIPTION OF Simple1.jj: This is a simple JavaCC grammar that recognizes a set of left braces followed by the same number of right braces and finally followed by zero or more line terminators and finally an end of file. Examples of legal strings in this grammar are: "{}", "{{{{{}}}}}", etc. Examples of illegal strings are: "{{{{", "{}{}", "{}}", "{{}{}}", etc. This grammar file starts with settings for all the options offered by JavaCC. In this case the option settings are their default values. Hence these option settings were really not necessary. One could as well have completely omitted the options section, or omitted one or more of the individual option settings. The details of the individual options is described in the JavaCC documentation in the web pages. Following this is a Java compilation unit enclosed between "PARSER_BEGIN(name)" and "PARSER_END(name)". This compilation unit can be of arbitrary complexity. The only constraint on this compilation unit is that it must define a class called "name" - the same as the arguments to PARSER_BEGIN and PARSER_END. This is the name that is used as the prefix for the Java files generated by the parser generator. The parser code that is generated is inserted immediately before the closing brace of the class called "name". In the above example, the class in which the parser is generated contains a main program. This main program creates an instance of the parser object (an object of type Simple1) by using a constructor that takes one argument of type java.io.InputStream ("System.in" in this case). The main program then makes a call to the non-terminal in the grammar that it would like to parse - "Input" in this case. All non-terminals have equal status in a JavaCC generated parser, and hence one may parse with respect to any grammar non-terminal. Following this is a list of productions. In this example, there are two productions, that define the non-terminals "Input" and "MatchedBraces" respectively. In JavaCC grammars, non-terminals are written and implemented (by JavaCC) as Java methods. When the non-terminal is used on the left-hand side of a production, it is considered to be declared and its syntax follows the Java syntax. On the right-hand side its use is similar to a method call in Java. Each production defines its left-hand side non-terminal followed by a colon. This is followed by a bunch of declarations and statements within braces (in both cases in the above example, there are no declarations and hence this appears as "{}") which are generated as common declarations and statements into the generated method. This is then followed by a set of expansions also enclosed within braces. Lexical tokens (regular expressions) in a JavaCC input grammar are either simple strings ("{", "}", "\n", and "\r" in the above example), or a more complex regular expression. In our example above, there is one such regular expression " " which is matched by the end of file. All complex regular expressions are enclosed within angular brackets. The first production above says that the non-terminal "Input" expands to the non-terminal "MethodBraces" followed by zero or more line terminators ("\n" or "\r") and then the end of file. The second production above says that the non-terminal "MatchedBraces" expands to the token "{" followed by an optional nested expansion of "MatchedBraces" followed by the token "}". Square brackets [...] in a JavaCC input file indicate that the ... is optional. [...] may also be written as (...)?. These two forms are equivalent. Other structures that may appear in expansions are: e1 | e2 | e3 | ... : A choice of e1, e2, e3, etc. ( e )+ : One or more occurrences of e ( e )* : Zero or more occurrences of e Note that these may be nested within each other, so we can have something like: (( e1 | e2 )* [ e3 ] ) | e4 To build this parser, simply run JavaCC on this file and compile the resulting Java files: javacc Simple1.jj javac *.java Now you should be able to run the generated parser. Make sure that the current directory is in your CLASSPATH and type: java Simple1 Now type a sequence of matching braces followed by a return and an end of file (CTRL-D on UNIX machines). If this is a problem on your machine, you can create a file and pipe it as input to the generated parser in this manner (piping also does not work on all machines - if this is a problem, just replace "System.in" in the grammar file with 'new FileInputStream("testfile")' and place your input inside this file): java Simple1 < myfile Also try entering illegal sequences such as mismatched braces, spaces, and carriage returns between braces as well as other characters and take a look at the error messages produced by the parser. --------------------------------------------------------------------- DETAILED DESCRIPTION OF Simple2.jj: Simple2.jj is a minor modification to Simple1.jj to allow white space characters to be interspersed among the braces. So then input such as: "{{ }\n}\n\n" will now be legal. Take a look at Simple2.jj. The first thing you will note is that we have omitted the options section. This does not change anything since the options in Simple1.jj were all assigned their default values. The other difference between this file and Simple1.jj is that this file contains a lexical specification - the region that starts with "SKIP". Within this region are 4 regular expressions - space, tab, newline, and return. This says that matches of these regular expressions are to be ignored (and not considered for parsing). Hence whenever any of these 4 characters are encountered, they are just thrown away. In addition to SKIP, JavaCC has three other lexical specification regions. These are: . TOKEN: This is used to specify lexical tokens (see next example) . SPECIAL_TOKEN: This is used to specify lexical tokens that are to be ignored during parsing. In this sense, SPECIAL_TOKEN is the same as SKIP. However, these tokens can be recovered within parser actions to be handled appropriately. . MORE: This specifies a partial token. A complete token is made up of a sequence of MORE's followed by a TOKEN or SPECIAL_TOKEN. Please take a look at some of the more complex grammars such as the Java grammars for examples of usage of these lexical specification regions. You may build Simple2 and invoke the generated parser with input from the keyboard as standard input. You can also try generating the parser with the various debug options turned on and see what the output looks like. To do this type: javacc -debug_parser Simple2.jj javac Simple2*.java java Simple2 Then type: javacc -debug_token_manager Simple2.jj javac Simple2*.java java Simple2 Note that token manager debugging produces a lot of diagnostic information and it is typically used to look at debug traces a single token at a time. --------------------------------------------------------------------- DETAILED DESCRIPTION OF Simple3.jj: Simple3.jj is the third and final version of our matching brace detector. This example illustrates the use of the TOKEN region for specifying lexical tokens. In this case, "{" and "}" are defined as tokens and given names LBRACE and RBRACE respectively. These labels can then be used within angular brackets (as in the example) to refer to this token. Typically such token specifications are used for complex tokens such as identifiers and literals. Tokens that are simple strings are left as is (in the previous examples). This example also illustrates the use of actions in the grammar productions. The actions inserted in this example count the number of matching braces. Note the use of the declaration region to declare variables "count" and "nested_count". Also note how the non-terminal "MatchedBraces" returns its value as a function return value. --------------------------------------------------------------------- DETAILED DESCRIPTION OF NL_Xlator.jj: This example goes into the details of writing regular expressions in JavaCC grammar files. It also illustrates a slightly more complex set of actions that translate the expressions described by the grammar into English. The new concept in the above example is the use of more complex regular expressions. The regular expression: < ID: ["a"-"z","A"-"Z","_"] ( ["a"-"z","A"-"Z","_","0"-"9"] )* > creates a new regular expression whose name is ID. This can be referred anywhere else in the grammar simply as . What follows in square brackets are a set of allowable characters - in this case it is any of the lower or upper case letters or the underscore. This is followed by 0 or more occurrences of any of the lower or upper case letters, digits, or the underscore. Other constructs that may appear in regular expressions are: ( ... )+ : One or more occurrences of ... ( ... )? : An optional occurrence of ... (Note that in the case of lexical tokens, (...)? and [...] are not equivalent) ( r1 | r2 | ... ) : Any one of r1, r2, ... A construct of the form [...] is a pattern that is matched by the characters specified in ... . These characters can be individual characters or character ranges. A "~" before this construct is a pattern that matches any character not specified in ... . Therefore: ["a"-"z"] matches all lower case letters ~[] matches any character ~["\n","\r"] matches any character except the new line characters When a regular expression is used in an expansion, it takes a value of type "Token". This is generated into the generated parser directory as "Token.java". In the above example, we have defined a variable of type "Token" and assigned the value of the regular expression to it. --------------------------------------------------------------------- DETAILED DESCRIPTION OF IdList.jj: This example illustrates an important attribute of the SKIP specification. The main point to note is that the regular expressions in the SKIP specification are only ignored *between tokens* and not *within tokens*. This grammar accepts any sequence of identifiers with white space in between. A legal input for this grammar is: "abc xyz123 A B C \t\n aaa" This is because any number of the SKIP regular expressions are allowed in between consecutive 's. However, the following is not a legal input: "xyz 123" This is because the space character after "xyz" is in the SKIP category and therefore causes one token to end and another to begin. This requires "123" to be a separate token and hence does not match the grammar. If spaces were OK within 's, then all one has to do is to replace the definition of Id to: TOKEN : { < Id: ["a"-"z","A"-"Z"] ( (" ")* ["a"-"z","A"-"Z","0"-"9"] )* > } Note that having a space character within a TOKEN specification does not mean that the space character cannot be used in the SKIP specification. All this means is that any space character that appears in the context where it can be placed within an identifier will participate in the match for , whereas all other space characters will be ignored. The details of the matching algorithm are described in the JavaCC documentation in the web pages. As a corollary, one must define as tokens anything within which characters such as white space characters must not be present. In the above example, if was defined as a grammar production rather than a lexical token as shown below this paragraph, then "xyz 123" would have been recognized as a legitimate (wrongly). void Id() : {} { <["a"-"z","A"-"Z"]> ( <["a"-"z","A"-"Z","0"-"9"]> )* } Note that in the above definition of non-terminal Id, it is made up of a sequence of single character tokens (note the location of <...>s), and hence white space is allowed between these characters. ---------------------------------------------------------------------