LESSON 387 OF 413 · PROJECTS: COMMAND-LINE TOOLS

CLI Tool: grep v4 — Whole Word Match in C#

Practice

Build the -w flag of the Unix grep command. Given a list of lines and a search pattern (a single word made of letters, digits, and underscores), return all lines that contain pattern as a WHOLE word — i.e. the characters immediately before and after the match must NOT be word characters (letters, digits, or underscores). The match is case-sensitive.

Return the matching lines joined by newline characters (\n).

Function signature:

public string GrepWord(List<string> lines, string pattern)

Example:

Input:
pattern = "cat"
lines = ["cat sat", "concat", "the cat", "category"]

Output: "cat sat\nthe cat"

When printed:
cat sat
the cat

Notice that "concat" is rejected because cat is preceded by the word character n (con|cat), and "category" is rejected because cat is followed by the word character e.

A line may contain the pattern more than once. It is selected if at least one occurrence is a whole word: with pattern "cat", the line "concat cat" is selected because of its second cat. The lines contain only ASCII characters.

If no lines match, return the empty string "".

Use System.Text.RegularExpressions.Regex with \b word-boundary anchors. Wrap the pattern itself with Regex.Escape to escape any regex metacharacters.

Regex regex = new Regex("\\b" + Regex.Escape(pattern) + "\\b");
// per line: if (regex.IsMatch(line)) keep

The full solution:

class Solution {
    public string GrepWord(List<string> lines, string pattern) {
        List<string> result = new List<string>();
        Regex regex = new Regex("\\b" + Regex.Escape(pattern) + "\\b");
        foreach (string line in lines) {
            if (regex.IsMatch(line)) {
                result.Add(line);
            }
        }
        return string.Join("\n", result);
    }
}

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CLI Tool: grep v4 — Whole Word Match in C#

Grep V4 in C#