i am having a lot of parameters defined in the lexer , whose input accepts values .

but one among the parameters should accept a string which may contain " alpha numeric characters along with a single white space in the input which may come at any place in the string ( for eg in first , middle or last )"

the parameter should accept the value only if it satisfies the above mentioned conditions otherwise it should throw an exception as
" invalid value for the parameter x ".

if anybody knows the solution let me know.
i have given the source code for which i have tried.
if anybody knows the answer let me know.

			

			header {
			

			

			import antlr.*;
			

			

			}
			

			

			class myparser extends Parser;
			

			

			options {
			

			

			k = 7;
			

			defaultErrorHandler = false;
			

			

			

			/*
			

			try {
			

			parser.startRule(); // parse as usual
			

			myparser parser=new myparser(filter);
			

			parser.setASTNodeClass("antlr.CommonASTwithHiddenTokens");
			

			

			}
			

			

			catch (Exception e) {
			

			System.err.println(e.getMessage());
			

			}
			

			*/
			

			

			

			}
			

			

			startRule throws Exception
			

			{
			

			System.out.println(parser);
			

			}
			

			:
			

			(createlexer
			

			)EOF
			

			;
			

			

			

			createlexer throws exception : (WS)?
			

			{
			

			mylexer lexer =new mylexer(System.in);
			

			

			lexer.setTokenObjectClass("mylexer");
			

			

			lexer.setTokenObjectClass("antlr.CommonHiddenStreamToken");
			

			

			TokenStreamHiddenTokenFilter filter=new TokenStreamHiddenTokenFilter(lexer);
			

			

			System.out.println(filter.getHiddenAfter(end));
			

			

			System.out.println(filter.getHiddenBefore(begin));
			

			

			filter.hide(myparser.WS);
			

			

			filter.hide(myparser.SL_COMMENT);
			

			

			myparser parser=new myparser(filter);
			

			

			}
			

			;
			

			

			class mylexer extends Lexer;
			

			options {
			

			

			k=10;
			

			

			caseSensitiveLiterals = false;
			

			caseSensitive = false;
			

			charVocabulary = ' '..'~';
			

			}
			

			

			/*
			

			mylexer lexer =new mylexer(System.in);
			

			

			lexer.setTokenObjectClass("mylexer");
			

			

			lexer.setTokenObjectClass("antlr.CommonHiddenStreamToken");
			

			

			TokenStreamHiddenTokenFilter filter=new TokenStreamHiddenTokenFilter(lexer);
			

			

			System.out.println(filter.getHiddenAfter(end));
			

			

			System.out.println(filter.getHiddenBefore(begin));
			

			

			filter.hide(myparser.WS);
			

			

			filter.hide(myparser.SL_COMMENT);
			

			

			

			*/
			

			

			

			DECLS:(decl)+;
			

			

			DECL:begin :INT ID end: SEMI
			

			;
			

			

			WS : (' ' | ' ')+;
			

			

			SL_COMMENT : ('/')+;