Valid Phone Numbers

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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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

✓ Manual Character Checking

⏱️ Time: O(n × m) Space: O(1)

Parse each line character by character to verify it matches either (xxx) xxx-xxxx or xxx-xxx-xxxx format exactly.

Regular Expression Pattern Matching

⏱️ Time: O(n × m) Space: O(1)

Create a single regular expression that matches both (xxx) xxx-xxxx and xxx-xxx-xxxx formats using alternation and digit matching.

Manual Character Checking — Algorithm Steps

Read each line from file
Check length and character positions
Verify digits are in correct positions

Visualization

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Step-by-Step Walkthrough

Check Length

Verify line is exactly 12 or 14 characters

Check Positions

Verify separators are in correct positions

Check Digits

Verify all number parts contain only digits

Code -

solution.c — C

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

int isDigits(const char* str, int start, int length) {
    for (int i = start; i < start + length; i++) {
        if (!isdigit(str[i])) return 0;
    }
    return 1;
}

void solution(const char* filename) {
    FILE* file = fopen(filename, "r");
    if (!file) return;
    
    char line[256];
    while (fgets(line, sizeof(line), file)) {
        int len = strlen(line);
        if (line[len-1] == '\n') {
            line[len-1] = '\0';
            len--;
        }
        
        if (len == 14 && line[0] == '(' && line[4] == ')' && line[5] == ' ' && line[9] == '-') {
            if (isDigits(line, 1, 3) && isDigits(line, 6, 3) && isDigits(line, 10, 4)) {
                printf("%s\n", line);
            }
        } else if (len == 12 && line[3] == '-' && line[7] == '-') {
            if (isDigits(line, 0, 3) && isDigits(line, 4, 3) && isDigits(line, 8, 4)) {
                printf("%s\n", line);
            }
        }
    }
    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 to check each position

✓ Linear Growth

Space Complexity

O(1)

Only variables for current line processing

✓ Linear Space

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Input & Output

Constraints

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Related Problems

Common Approaches

Manual Character Checking — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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