Ramiro N. Cosa

Sentence Capitalizer - FreeCodeCamp #37 Daily Challenge

2 min

Sentence Capitalizer - Analysis and Explanation

Problem Statement

Given a paragraph, the goal is to return a new string where the first letter of each sentence is capitalized.

  • All other characters must remain the same.
  • Sentences can end with a period (.), one or more question marks (?), or one or more exclamation points (!).

Initial Analysis

Understanding the Problem

The challenge consists of identifying the start of each sentence within a block of text. A sentence begins in two possible situations:

  1. At the beginning of the paragraph.
  2. Immediately after a final punctuation mark (., ?, !), possibly with spaces in between.

We must transform only the first alphabetic character of each sentence to uppercase, keeping the rest of the text intact.

Process Visualization

flowchart TD
  A["Start of Paragraph"] --> B{"Is lowercase letter?"}
  B -- "Yes" --> C["Capitalize"]
  B -- "No" --> D["Keep as is"]
  E["Punctuation Mark (.?!)"] --> F["Optional: Spaces"]
  F --> G{"Is next a letter?"}
  G -- "Yes" --> C
  G -- "No" --> D

Identified Test Cases

  1. Start of paragraph: this is... →\rightarrow This is...
  2. Multiple sentences with spaces: hello world. how are you? →\rightarrow Hello world. How are you?
  3. Repeated punctuation (ellipsis/exclamations): challenge... it was fun!! →\rightarrow challenge... It was fun!!
  4. No spaces between sentences: crazy!!!strange →\rightarrow Crazy!!!Strange
  5. Spaces before punctuation: . why →\rightarrow . Why

Solution Development

Chosen Approach

The most efficient and readable way to solve this is using Regular Expressions (RegExp). We will use JavaScript/TypeScript’s replace() method with a callback function to transform only the captured letter.

The RegExp should look for:

  • The start of the string (^).
  • Or a sequence of final punctuation marks ([.?!]+) followed optionally by spaces (\s*).
  • Followed by a lowercase letter ([a-z]).

Implementation

/**
 * Capitalizes the first letter of each sentence in a paragraph.
 * @param paragraph The input text.
 * @returns The text with capitalized sentences.
 */
function capitalizeSentences(paragraph: string): string {
  // RegExp Explanation:
  // (^|[.?!]+\s*) -> Group 1: Start of line OR final punctuation + optional spaces
  // ([a-z])       -> Group 2: The first lowercase letter that follows
  const regex = /(^|[.?!]+\s*)([a-z])/g

  return paragraph.replace(regex, (match, p1, p2) => {
    return p1 + p2.toUpperCase()
  })
}

Complexity Analysis

Time Complexity

O(n)O(n), where nn is the length of the paragraph. The regular expression engine traverses the string once to identify and replace matches.

Space Complexity

O(n)O(n). A new string is created for the result, with a size proportional to the input string.

Edge Cases and Considerations

  • Multiple punctuation: Using + in [.?!]+ correctly handles cases like ... or !!!.
  • Absence of spaces: The \s* expression allows sentences to be capitalized even if there is no space after the period.
  • Non-alphabetic characters: If a sentence starts with a number or symbol, the RegExp will not match [a-z], which is correct since they cannot be capitalized.

Reflections and Learnings

Applied Concepts

  • Capturing Groups: Allow separating the “anchor” of the sentence (punctuation/start) from the character we actually want to modify.
  • Regex Alternation (|): Fundamental for handling the special case of the start of the string along with sentence delimiters.

Resources and References

freecodecamp daily-challenge