# CS252: Systems Programming Ninghui Li Topic 4: Regular Expressions and Lexical Analysis.

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CS252: Systems Programming Ninghui Li Topic 4: Regular Expressions and Lexical Analysis

slide 2 Compiler Frontend Steps Lexical analyzer/scanner convert sequence of characters to sequence of tokens (inc 13) becomes 4 tokens, (, inc, 13, ) Parser/syntactic analysis analyze a sequence of tokens to create/determine the grammatical structure

slide 3 Brief Description of the Lab Part 1: Implement FIZ without user-defined functions (50%), due Feb 9 Part 2: Implement user-defined functions (50%), due Feb 16 Part 2 is significant harder than Part 1. Do not wait until the last week.

4 Using Lex/Flex with YACC/Bison

slide 5 Files Provided: fiz.l "inc" {return INC;} "(" {return OPENPAR;} ")" {return CLOSEPAR;} 0|[1-9][0-9]* { yylval.number_val = atoi(yytext); return NUMBER; } [ \t\n] {/* Discard spaces, tabs, and new lines */}.{printf("Syntax error. Did not recognize %s\n", yytext); }

slide 6 Files Provided: fiz.y /******************************************************* * Section 1: Definition of tokens and non-terminals * *****************************************************/ %token NUMBER %token INC OPENPAR CLOSEPAR %type expr %union{ char *string_val; intnumber_val; struct TREE_NODE *node_val; } The NUMBER token has number_value These three tokens have no value A parsed expr has a pointer to a node in an Abstract Syntax Tree associated with it. This defines the union associated with each token or non-terminal when parsing.

slide 7 Files Provided: fiz.y /************************************************** * Section 3: Grammar production rules * **************************************************/ goal: statements; statements: statement | statement statements; statement: expr { err_value = 0;resolve(\$1, NULL); if (err_value == 0) { printf ("%d\n", eval(\$1, NULL)); } prompt(); }; Red code are currently unnecessary. They are needed when user-defined functions are implemented. Green code evaluates the expression. \$1 refers to the AST node associated with the 1 st element in the grammar rule, namely expr

slide 8 Abstract Syntax Tree A abstract syntax tree, is a tree representation of the abstract syntactic structure of source code written in a programming language. Each node of the tree denotes a construct occurring in the source code. The syntax is "abstract" in not representing every detail appearing in the real syntax. For instance, grouping parentheses are implicit in the tree structure, and a syntactic construct like an if-condition-then expression may be denoted by means of a single node with three branches.

slide 9 Abstract Syntax Tree: An Example IFZ_NODE ARG_NAME strValue = “y” ARG_NAME strValue = “x” FUNC_CALL name =“add” INC_NODE ARG_NAME strValue = “x” DEC_NODE ARG_NAME strValue = “y” The above is an AST for (ifz y x (add (inc x) (dec y))), The body of the function (add x y) Consider how evaluate (add 4 1) would work.

slide 10 Grammar for expr expr: OPENPAR INC expr CLOSEPAR { struct TREE_NODE * node = (struct TREE_NODE *) malloc(sizeof(struct TREE_NODE)); node -> type = INC_NODE; node -> first_arg = \$3; \$\$ = node; } The above production rule (grammar rule) parses (inc ) It creates a node in the abstract syntax tree, denote its type to be INC_NODE, and stores the tree node for in first_arg; since this is the first (and only) argument of (inc ). \$3 refers to the value associated with the 3 rd element in the grammar, i.e., expr in the body \$\$ refers to the value associated with expr on the left hand side

slide 11 Continuing grammar for expr | NUMBER { struct TREE_NODE * node = (struct TREE_NODE *) malloc(sizeof(struct TREE_NODE)); node -> type = NUMBER_NODE; node -> intValue = \$1; \$\$ = node; }; The above production rule (grammar rule) parses a number into an expr. It creates a node in the abstract syntax tree, denote its type to be NUMBER_NODE, and stores the integer value in the intValue field. \$1 refers to the value associated with the 1st element in the grammar, i.e., NUMBER in the body \$\$ refers to the value associated with expr on the left hand side

slide 12 What happens from Parsing? Input(inc (inc 1)) Becomes tokens: OPENPAR INC OPENPAR INC NUMBER CLOSEPAR CLOSEPAR This is parsed into statement in the following steps: statement: expr expr: OPENPAR INC expr CLOSEPAR expr: OPENPAR INC NUMBER CLOSEPAR INC_NODE NUMBER_NODE intValue = 1 INC_NODE

slide 13 Regular Expressions: Tool for Lexical Analyzer Regular expression: A notation to specify a pattern that matches a set of strings A regular expression can be: a a single character R 1 |R 2 matches anything that matches either R 1 or R 2 (R) matches the same thing as R [abcde] any of the five letter listed there, i.e., a|b|c|d|e [0-9] any digit

slide 14 Regular Expressions R 1 R 2 matches a string s if s is concatenation of s 1 s 2, and s 1 matches R 1 and s 2 matches R 2 E.g., [abcde] [0-9] matches R* repeating the regular expression R zero or more times E.g., [0-9]* matches the empty string and any digit sequence R+ repeating R one or more times Equivalent to the regular expression R R*

slide 15 RE Syntax in Lex/Flex http://flex.sourceforge.net/manual/Patterns.html ‘x’ match the character 'x' ‘.’ any character (byte) except newline ‘[xyz]’ a character class; in this case, the pattern matches either an 'x', a 'y', or a 'z' ‘[abj-oZ]’ a "character class" with a range in it; matches an 'a', a 'b', any letter from 'j' through 'o', or a 'Z' ‘[^A-Z]’ a "negated character class", i.e., any character but those in the class. In this case, any character EXCEPT an uppercase letter.

slide 16 RE Syntax in Lex/Flex http://flex.sourceforge.net/manual/Patterns.html ‘[^A-Z\n]’ any character EXCEPT an uppercase letter or a newline ‘[a-z]{-}[aeiou]’ the lowercase consonants ‘r*’ zero or more r's, where r is any regular expression ‘r+’ one or more r's ‘r?’ zero or one r's (that is, “an optional r”) ‘r{2,5}’ anywhere from two to five r's ‘r{2,}’ two or more r's ‘r{4}’ exactly 4 r's

slide 17 RE Syntax in Lex/Flex http://flex.sourceforge.net/manual/Patterns.html ‘{name}’ the expansion of the ‘name’ definition ‘"[xyz]\"foo"’ the literal string: ‘[xyz]"foo’ ‘(r)’ match an ‘r’; parentheses are used to override precedence ‘rs’ the regular expression ‘r’ followed by the regular expression ‘s’; called concatenation ‘r|s’ either an ‘r’ or an ‘s’ ‘^r’ an ‘r’, but only at the beginning of a line ‘r\$’ an ‘r’, but only at the end of a line

slide 18 Examples Regular expression for an non-negative integer: Is [0-9]* correct? Yes, if allowing 00123 is okay, 0 | [1-9][0-9]* is better Regular expression for an identifier: Rule 1: Name of identifier includes alphabets and digits. Rule 2: First character of any identifier must be a letter. How to write the regular expression? [a-zA-Z][a-zA-Z0-9]*

19 More Questions on RE How to write regular expression that matches comments, assuming that comments are defined as anything between ; and end of line?

20 Review Able to write simple regular expressions to match strings. Given a regular expression, able to tell what are matched are what are not.

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