Use awk to parse a expression

parser.awk

#!/bin/awk
# input: a:=10 
# output： the value is 10

BEGIN{ PrintRes() }

function parse() {
	if(tok == "(eof)") return "(eof)"
	while(length(line) == 0)
		if(getline line == 0)
			return tok = "(eof)"
	sub(/^[ \t]+/,"",line)	
	if(match(line,/^[A-Za-z_][A-Za-z_0-9]*/) || match(line,/^(=|:=)/) || 
	   match(line,/^-?([0-9]+\.?[0-9]*|\.[0-9]+)/) || match(line,/^./)){
		   tok = substr(line,1,RLENGTH)
		   line = substr(line,RLENGTH+1)
   		   return tok
   }
}


function PrintRes(){
		parse()
		while(tok != "(eof)"){
			if(temp == ":=")
					value = tok
			temp = tok
			parse()
	}
	print "The value is", value
}