CMP 335 Regular Expression Exercises Note PDF

**Introduction** Regular expressions (Regex) are a powerful tool used in various fields of computer science, including compiler design. They provide a concise and flexible means of describing patterns in text, which is essential for tasks like lexical analysis in compiler design. This blog will del...

**Introduction** Regular expressions (Regex) are a powerful tool used in various fields of computer science, including compiler design. They provide a concise and flexible means of describing patterns in text, which is essential for tasks like lexical analysis in compiler design. This blog will delve into the significance of regular expressions in compiler design, explaining their role, usage, and impact on the overall process of compiling a program. ARegular Expression in Compiler Design **What are Regular Expressions?** Regular expressions are sequences of characters that define search patterns, primarily used for string matching within texts. These patterns can be simple, such as matching a single character, or complex, involving combinations of various characters, special symbols, and operators. **Basic Syntax of Regular Expressions** - - - - - - - **Examples** - - **Role of Regular Expressions in Compiler Design** In compiler design, regular expressions play a crucial role in the **lexical analysis** phase, which is the first phase of a compiler. The primary task of lexical analysis is to read the source code and convert it into tokens, which are the smallest units of meaning (like keywords, operators, and identifiers). **Lexical Analysis** The lexical analyzer, also known as the **scanner**, uses regular expressions to identify patterns in the source code and classify them into tokens. These tokens are then used by the parser in the subsequent phase of the compiler. For example, consider the following piece of code: **int main() {\ int a = 5;\ }** The lexical analyzer would break this code into tokens like int, main, (, ), {, }, a, =, 5, ;. To identify each of these tokens, the lexer relies on regular expressions: - **Keywords** like int and main can be matched directly using literals or predefined patterns. - **Identifiers** (like variable names) can be matched using regex that allows sequences of letters and digits. - **Operators** like = can be matched using specific literals. **Conversion to Finite Automata** Regular expressions are not only used for pattern matching but can also be converted into finite automata, which are used to recognize tokens. This conversion is an essential step in the lexical analysis process. - **Deterministic Finite Automata (DFA):** A DFA is used to recognize tokens in a single pass over the input string. It's efficient and suitable for real-time processing. - **Nondeterministic Finite Automata (NFA):** An NFA is more flexible in terms of pattern matching but requires more complex processing, as it can be in multiple states simultaneously. The process usually involves converting the regular expression into an NFA, which is then optimized and transformed into a DFA. This DFA can then be used by the lexer to scan the source code efficiently. - - - - - **Conclusion** Regular expressions are a fundamental tool in compiler design, particularly in the lexical analysis phase. They enable the efficient identification and classification of tokens in source code, facilitating the compilation process. By converting regular expressions into finite automata, compilers can quickly process and recognize patterns, ensuring that the source code is correctly parsed and transformed into executable code. Despite their limitations, regular expressions remain an essential component in the toolkit of compiler designers. **QUESTIONS AND SOLUTIONS** 1. What is Operator Precedence Parsing Algorithm in compiler design? ==================================================================== Any string of Grammar can be parsed by using stack implementation, as in shift Reduce parsing. But in operator precedence parsing shifting and reducing is done based on Precedence Relation between symbol at the top of stack & current input symbol of the input string to be parsed. The operator precedence parsing algorithm is as follows − **Input** − The precedence relations from some operator precedence grammar and an input string of terminals from that grammar. **Output** − There is no output but it can construct a skeletal parse tree as we parse, with one non-terminal labeling all interior nodes and the use of single productions not shown. Alternatively, the sequence of shift-reduce steps can be considered the output. **Method** − Let the input string be a~1~a~2~ ....... a~n~.\$Initially, the stack contains \$. - Repeat forever - If only \$ is on the stack and only \$ is on the input then accept and break else - begin - let a be the topmost terminal symbol on the stack and let b be the current input symbols. - If a \b then /\*reduce\*/ - repeat pop the stack - until the top stack terminal is related by \.\> \ \.\>.\>.\> \.\>.\>.\> \.\>.\>.\>.\>.\>.\> ( \ \$ \ \$ \=2} is − S→bbB    ⇒for first 2 b's B→bB\|aC  ⇒ any number of b's followed by a C→bbD    ⇒ 2b's D→ bD\|a  ⇒ any number of b's followed by a 10.Explain about left linear regular grammar in TOC =================================================== Regular grammar describes a regular language. It consists of four components, which are as follows − G = (N, E, P, S) Where, - N − finite set of non-terminal symbols, - E − a finite set of terminal symbols, - P − a set of production rules, each of one is in the forms - S → aB - S → a - S → ∈, - S ∈ N is the start symbol. The above grammar can be of two forms − - Right Linear Regular Grammar - Left Linear Regular Grammar **Linear Grammar** ------------------ When the right side of the Grammar part has only one terminal then it\'s linear else nonv linear. **eft linear grammar** ---------------------- In a left-regular grammar (also called left-linear grammar), the rules are of the form as given below − - L → a, {L is a non-terminal in N and a is a terminal in Σ} - L → Ma, {L and M are in N and a is in Σ} - L → ∈, {∈ is the empty string}. The left linear grammar means that the non-terminal symbol will be at the left side. ### **Example** Consider a language {b^n^ab^m^a\| n\>=2, m\>=2} The left linear grammar that is generated based on given language is − S → Bbba       ⇒ last 3 symbols bba B → Bb\| Dbba   ⇒ for b^m^ and bba are for bn followed by a. D → Db\|e       ⇒ for b^n-2^