Valid Phone Numbers - Practice Coding Problems

Valid Phone Numbers - Problem

Shell Easy

Given a text file file.txt that contains a list of phone numbers (one per line), write a one-liner bash script to print all valid phone numbers.

You may assume that a valid phone number must appear in one of the following two formats:

(xxx) xxx-xxxx
xxx-xxx-xxxx

where x means a digit. You may also assume each line in the text file must not contain leading or trailing white spaces.

Input & Output

Example 1 — Mixed Format File

$ Input: file.txt contains: (555) 123-4567 555-123-4567 555.123.4567 5551234567

› Output: (555) 123-4567 555-123-4567

💡 Note: Only the first two lines match valid formats: (xxx) xxx-xxxx and xxx-xxx-xxxx

Example 2 — All Valid Numbers

$ Input: file.txt contains: (123) 456-7890 987-654-3210 (999) 888-7777

› Output: (123) 456-7890 987-654-3210 (999) 888-7777

💡 Note: All three lines match valid phone number formats

Example 3 — No Valid Numbers

$ Input: file.txt contains: 555.123.4567 555 123 4567 +1-555-123-4567

› Output:

💡 Note: None of these formats match the required patterns

Constraints

Each line contains at most one phone number
No leading or trailing whitespace on lines
Valid formats: (xxx) xxx-xxxx or xxx-xxx-xxxx where x is a digit

Visualization

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Understanding the Visualization

Input File

Text file with mixed phone number formats

Pattern Match

Apply regex to identify valid formats

Output

Print only lines matching (xxx) xxx-xxxx or xxx-xxx-xxxx

Key Takeaway

🎯 Key Insight: Regular expressions provide a concise way to match multiple string patterns simultaneously

Asked in

a Amazon 15 M Microsoft 8

The key insight is using regular expressions to match both phone number formats in a single pattern. The optimal approach uses grep -E '^($[0-9]{3}$ [0-9]{3}-[0-9]{4}|[0-9]{3}-[0-9]{3}-[0-9]{4})$' file.txt as a one-liner bash command. Time: O(n×m), Space: O(1)

Common Approaches

Approach	Time	Space	Notes
✓ Regular Expression Pattern Matching	O(n × m)	O(1)	Use regex pattern to match both valid phone number formats in one expression
Manual Character Checking	O(n × m)	O(1)	Check each character position manually for valid formats

Regular Expression Pattern Matching — Algorithm Steps

Use grep with regex pattern
Match either format with alternation |
Return all matching lines

Visualization

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Step-by-Step Walkthrough

Pattern Creation

Build regex: ^($[0-9]{3}$ [0-9]{3}-[0-9]{4}|[0-9]{3}-[0-9]{3}-[0-9]{4})$

Pattern Matching

Test each line against the regex pattern

Output Matches

Print lines that match either format

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <regex.h>

void solution(const char* filename) {
    FILE* file = fopen(filename, "r");
    if (!file) return;
    
    regex_t regex;
    int reti = regcomp(&regex, "^(\\([0-9]\\{3\\}\\) [0-9]\\{3\\}-[0-9]\\{4\\}\\|[0-9]\\{3\\}-[0-9]\\{3\\}-[0-9]\\{4\\})$", REG_EXTENDED);
    if (reti) return;
    
    char line[256];
    while (fgets(line, sizeof(line), file)) {
        int len = strlen(line);
        if (line[len-1] == '\n') {
            line[len-1] = '\0';
        }
        
        reti = regexec(&regex, line, 0, NULL, 0);
        if (!reti) {
            printf("%s\n", line);
        }
    }
    
    regfree(&regex);
    fclose(file);
}

int main() {
    char filename[256];
    fgets(filename, sizeof(filename), stdin);
    filename[strcspn(filename, "\n")] = 0;
    
    solution(filename);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n × m)

n lines × m characters per line for regex matching

✓ Linear Growth

Space Complexity

O(1)

Constant space for regex engine state

✓ Linear Space

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Regular Expression Pattern Matching — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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