1. Syntax One - Hybrid
CMSC 331

2. Parse Trees What’s the parse tree for this statement ?
PS → P |
P PS
P → e |
'(' PS ')' |
'<' PS '>' |
'[' PS ']'
What’s the parse
tree for this statement ?
< [ ] [ < > ] >

3. Derivation for < [ ] [ < > ] >
PS → P
→ < PS >
→ P PS >
→ < [ PS ] PS >’
→ < [P] PS >’
→ < [ e] PS >
→ <‘[ ] PS >
→ < [ ] P >
→ < [ ] [ PS ] >
→ < [ ] [ P ] >
→ < [ ] [‘<PS > ] >
→ < [ ] [‘<P > ] >’
→ < [ ] [< e ’> ] >
→ < [ ] [ < > ] >

4. Ambiguity Two parse trees for the same expression
Consider the following grammar
String → String + String |
String - String | DIGIT
What are the two trees for

5. Parse Tree 1
String +
DIGIT(2)
DIGIT(9) -
DIGIT(5)

6. Parse Tree 2
String
DIGIT(9) -
DIGIT(5)
DIGIT(2) *

7. EBNF - Extended BNF Like BNF except that
Non-terminals start w/ uppercase
Parens are used for grouping terminals
Braces {} represent zero or more occurrences (iteration )
Brackets [] represent an optional construct , that is a construct that appears either once or not at all.

8. EBNF example Exp → Term { ('+' | '-') Term }
Term → Factor { ('*' | '/') Factor }
Factor → '(' Exp ')' | variable | constant

9. EBNF/BNF EBNF and BNF are equivalent How can {} be expressed in BNF?

10. EBNF for {} BNF EBNF for [] X-> a { b } g X -> a X’
X’ -> b X’ | g
EBNF for []
X-> a [b ] g
X -> a b g | a g

11. Syntax Graphs Terminal in circles Non- terminals in rectangles;
Syntax Graphs - put the terminals in circles or
ellipses and put the nonterminals in rectangles;
connect with lines with arrowheads
e.g., Pascal type declarations
type_identifier
( identifier ) ,
constant constant

12. Recursive Descent An easy way to build a parser Example
Does work in the face of left recursion
Purge left recursion