Week 5 - C Regex & C Fundamentals Part 3

Regex in C

Compiling Regex

• Agenda: Overview of regex compilation in C.

• Basic Patterns:

  • Phone Number Format: ###-###-####

    • Regex: ^\d{3}-\d{3}-\d{4}$

  • Date Format: MM/DD/YYYY

    • Regex: ^(0[1-9]|1[0-2])\/(0[1-9]|[12]\d|3[01])\/\d{4}$

  • Password Criteria: At least one upper case, one lower case, one digit

    • Regex: ^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).+$

Regular Expression Engines

• Definition: Software that processes regex patterns.

• Common Engines: Perl, PHP, Java, JavaScript, Python, R, POSIX.

• C Implementation: Uses POSIX regex syntax, compatible with UNIX tools (grep, egrep, awk, sed).

Regex Functions in C

• Library Used: <regex.h>

• Functions:

  • regcomp()

    • Compiles the regex pattern.

  • regexec()

    • Executes the compiled pattern against a string.

• Compilation Command in gcc: gcc my_prog.c -o my_prog -lregex

Compiling and Executing Regex

• regcomp():

  • Takes 3 arguments: regex variable, pattern, and special flags (e.g., REG_EXTENDED).

• regexec():

  • Takes 5 arguments: compiled regex, string, number of sub-expressions, and offset array.

Example

• Valid Email Check:

  • Regex: ^([a-zA-Z0-9_\-.]+)@([a-zA-Z0-9_\-.]+)\.([a-zA-Z]{2,5})$

C Fundamentals

Expressions

• Arithmetic Operators: +, -, *, /, %, ++, --

• Relational Operators: <, >, <=, >=, != (returns 0 for false, 1 for true in C)

• Conditional Operators: &&, ||, ?

• Assignment Operators: =, +=, -=, *=, /=, &=, |=, ^=

• Unary Operators: * (pointer dereference), & (address), sizeof (size of type), -> (field selection)

• Bitwise Operators:

  • And (&), Or (|), Exclusive Or (^), Left Shift (<<), Right Shift (>>), Complement (~)

Scope Rules

• Local Variables:

  • Declared in functions, lifetime = function execution, scope = function body.

• Global Variables:

  • Declared outside functions, lifetime = program duration, scope = entire file.

• Static Variables:

  • Use static prefix to limit scope; persistent storage upon function exit.

Example of Scope

#include <stdio.h>

double pi = 3.14159; // Global variable

int main(void) {
    char user_input[30]; // Local variable
    scanf("%s", user_input); // Read user input
}

Next Class

• Topic: File I/O in C