string_editor

A Gleam library for string manipulation and extraction. Extract substrings before, after, or between specific patterns.

Installation

gleam add string_editor

Usage

import string_editor

pub fn main() -> Nil {
  // Extract text before a pattern
  let assert Ok("hello") = string_editor.before("hello world", on: " ")
  
  // Extract text after a pattern  
  let assert Ok("world") = string_editor.after("hello world", on: " ")
  
  // Extract text between two patterns
  let assert Ok("content") = string_editor.between("<div>content</div>", from: "<div>", to: "</div>")
  
  // Count occurrences of a pattern
  let count = string_editor.count("hello hello world", of: "hello") // 2
  
  // Extract at specific index
  let assert Ok("a.b") = string_editor.before_at("a.b.c.d", on: ".", at: 1)
  
  // Extract all occurrences
  let all_before = string_editor.before_all("a.b.c.d", on: ".") // ["a", "a.b", "a.b.c"]
}

API Reference

before(string: String, on pattern: String) -> Result(String, Nil)

Returns the part of a string before the first occurrence of a given substring.

Examples:

string_editor.before("hello world", on: " ")
// Ok("hello")

string_editor.before("no-match", on: "!")  
// Error(Nil)

after(string: String, on pattern: String) -> Result(String, Nil)

Returns the part of a string after the first occurrence of a given substring.

Examples:

string_editor.after("hello world", on: " ")
// Ok("world")

string_editor.after("no-match", on: "!")
// Error(Nil)

between(string: String, from start: String, to end: String) -> Result(String, Nil)

Returns the part of a string between two given substrings. Finds the first occurrence of start and then the first occurrence of end after start.

Examples:

string_editor.between("<a>link</a>", from: "<a>", to: "</a>")
// Ok("link")

string_editor.between("<h1>title</h1>", from: "<h1>", to: "</h2>")
// Error(Nil)

count(string: String, of pattern: String) -> Int

Counts the number of occurrences of a substring in a string.

Examples:

string_editor.count("hello hello world", of: "hello")
// 2

string_editor.count("gleam is fun", of: "rust")
// 0

string_editor.count("aaaa", of: "aa")
// 2 (non-overlapping matches)

before_at(string: String, on pattern: String, at index: Int) -> Result(String, Nil)

Returns the part of a string before the nth occurrence of a given substring (0-indexed).

Examples:

string_editor.before_at("a.b.c.d", on: ".", at: 1)
// Ok("a.b")

string_editor.before_at("hello world", on: " ", at: 5)
// Error(Nil)

after_at(string: String, on pattern: String, at index: Int) -> Result(String, Nil)

Returns the part of a string after the nth occurrence of a given substring (0-indexed).

Examples:

string_editor.after_at("a.b.c.d", on: ".", at: 1)
// Ok("c.d")

string_editor.after_at("hello world", on: " ", at: 5)
// Error(Nil)

between_at(string: String, from start: String, to end: String, at index: Int) -> Result(String, Nil)

Returns the part of a string between the nth occurrence of start and the first occurrence of end after that (0-indexed for start pattern).

Examples:

string_editor.between_at("<a>1</a><a>2</a>", from: "<a>", to: "</a>", at: 1)
// Ok("2")

string_editor.between_at("<h1>title</h1>", from: "<h1>", to: "</h2>", at: 0)
// Error(Nil)

before_all(string: String, on pattern: String) -> List(String)

Returns all parts of a string before each occurrence of a given substring.

Examples:

string_editor.before_all("a.b.c.d", on: ".")
// ["a", "a.b", "a.b.c"]

string_editor.before_all("hello world", on: "!")
// []

after_all(string: String, on pattern: String) -> List(String)

Returns all parts of a string after each occurrence of a given substring.

Examples:

string_editor.after_all("a.b.c.d", on: ".")
// ["b.c.d", "c.d", "d"]

string_editor.after_all("hello world", on: "!")
// []

between_all(string: String, from start: String, to end: String) -> List(String)

Returns all parts of a string between each occurrence of start and the next occurrence of end.

Examples:

string_editor.between_all("<a>1</a><b>2</b><a>3</a>", from: "<a>", to: "</a>")
// ["1", "3"]

string_editor.between_all("no matches here", from: "<div>", to: "</div>")
// []

Common Use Cases

HTML/XML Parsing

// Extract content from HTML tags
string_editor.between("<title>My Page</title>", from: "<title>", to: "</title>")
// Ok("My Page")

// Extract all link texts from HTML
string_editor.between_all("<a>Home</a> <a>About</a> <a>Contact</a>", from: "<a>", to: "</a>")
// ["Home", "About", "Contact"]

// Count div tags in HTML
string_editor.count("<div>content</div><div>more</div>", of: "<div>")
// 2

File Path Manipulation

// Get filename from path
string_editor.after("/home/user/document.txt", on: "/")
// Ok("document.txt")

// Get file extension
string_editor.after("document.txt", on: ".")
// Ok("txt")

// Get all directory components
string_editor.after_all("/home/user/projects/myapp", on: "/")
// ["home/user/projects/myapp", "user/projects/myapp", "projects/myapp", "myapp"]

// Count directory levels
string_editor.count("/home/user/projects/myapp", of: "/")
// 4

URL Parsing

// Extract domain from URL
string_editor.between("https://example.com/path", from: "://", to: "/")
// Ok("example.com")

Configuration Parsing

// Extract values from key=value pairs
string_editor.after("DATABASE_URL=postgres://localhost", on: "=")
// Ok("postgres://localhost")

// Parse all environment variables from a string
string_editor.after_all("PORT=3000\nDB_HOST=localhost\nDB_PORT=5432", on: "=")
// ["3000\nDB_HOST=localhost\nDB_PORT=5432", "localhost\nDB_PORT=5432", "5432"]

// Count configuration entries
string_editor.count("key1=value1,key2=value2,key3=value3", of: "=")
// 3

Log Processing

// Extract all timestamps from logs
string_editor.before_all("2023-01-01 INFO: message\n2023-01-02 ERROR: problem", on: " INFO:")
// Would extract timestamp parts before INFO entries

// Count error occurrences
string_editor.count("INFO: ok\nERROR: fail\nINFO: ok\nERROR: fail", of: "ERROR:")
// 2

Error Handling

Functions have different return types based on their purpose:

Result Functions

Functions that return Result(String, Nil) return Error(Nil) when:

• The pattern is not found in the string (before, after, between)
• The pattern doesn't occur enough times (before_at, after_at, between_at)
• For between functions, when either the start or end pattern is not found in the correct order

Count Function

count() always returns an Int (never fails), returning 0 when no matches are found.

List Functions

*_all functions always return a List(String) (never fail), returning an empty list [] when no matches are found.

Performance Analysis

Here's an analysis of the performance characteristics of each function:

before() and after() Functions

Time Complexity: O(n) where n is the length of the input string

• Uses string.split_once() which performs a single pass through the string
• Stops at the first occurrence of the pattern
• Minimal string allocations for the result

Space Complexity: O(k) where k is the length of the result substring

• Returns only the required portion of the string
• Minimal intermediate allocations
• Memory usage primarily scales with output size

Performance Characteristics:

• Best case: Pattern found early in string - O(p) where p is position of pattern
• Worst case: Pattern not found - O(n) full string scan
• Memory usage: Utilizes Gleam's standard string operations

between() Function

Time Complexity: O(n) where n is the length of the input string

• Makes two sequential calls to the underlying split operations
• First finds the start pattern, then searches the remainder for the end pattern
• Still linear overall as each character is examined at most twice

Space Complexity: O(k) where k is the length of the extracted content

• Creates one intermediate string (the portion after the start pattern)
• Final result is a substring of that intermediate string
• Memory usage remains proportional to output, not total input

Performance Characteristics:

• Best case: Both patterns found early - O(p₁ + p₂) where p₁, p₂ are pattern positions
• Worst case: End pattern not found - O(n) where n is length after start pattern
• Implementation: Built on top of the after() and before() functions

count() Function

Time Complexity: O(n) where n is the length of the input string

• Uses string.split() which performs a single pass through the string
• Counts splits by getting list length and subtracting 1
• Handles edge cases (empty patterns) in constant time

Space Complexity: O(m) where m is the number of splits

• Creates a list of string parts during splitting
• Memory scales with both the number of pattern occurrences and the size of the split parts
• No regex compilation overhead for simple pattern matching

Performance Characteristics:

• Best case: Pattern not found - O(n) scan with minimal memory
• Worst case: Many small patterns - O(n) time but higher memory for split results
• Counting approach: Gets list length rather than iterating through results

Indexed Functions (*_at)

Time Complexity: O(n) where n is the length of the input string

• All use string.split() for initial parsing - single pass through string
• List operations (take, drop, join) are O(m) where m is number of splits
• Overall complexity remains O(n) as splits are bounded by string length

Space Complexity: O(m) where m is the number of parts after splitting

• Creates list of all split parts, even if only using subset
• Result size is O(k) where k is length of extracted content
• Uses more memory than basic functions when there are many pattern matches

Performance Characteristics:

• Best case: Low index with early patterns - O(n) time, minimal extra memory
• Worst case: High index with many splits - O(n) time, O(m) space for all parts
• Index validation: Bounds checking happens before processing

Multi-Instance Functions (*_all)

Time Complexity: O(n + m²) where n is string length, m is number of splits

• Initial split operation: O(n)
• For each result position (m-1 results), rebuilds string from parts: O(m)
• Overall: O(n + m²) where m is typically much smaller than n

Space Complexity: O(m × k) where m is matches, k is average result length

• Stores all results in a list
• Each result requires reconstructing string from parts
• Memory scales with both number of matches and their sizes

Performance Characteristics:

• Best case: Few patterns, short results - approaches O(n)
• Worst case: Many patterns creating large results - O(n + m²) time, O(m × k) space
• Batch processing: Single split operation shared across all results

between_all() Function

Time Complexity: O(n + m² + r) where n is input length, m is start matches, r is total results

• Leverages after_all() for start pattern extraction: O(n + m²)
• Filters each result through before(): O(r) where r ≤ m
• Combined complexity: O(n + m² + r)

Space Complexity: O(m × k + r × j) where k is average after_all result size, j is final result size

• Intermediate storage for all after_all results
• Final filtered results list
• Memory peaks during intermediate step, then reduces after filtering

Performance Characteristics:

• Best case: Few start patterns, most have matching end patterns - O(n + m²)
• Worst case: Many start patterns, few matching end patterns - O(n + m²) time, with higher intermediate memory usage
• Filtering approach: Built-in filtering reduces final memory footprint

Real-World Performance Implications

Suitable for simple use cases involving:

• Log parsing: Extract basic timestamps, error codes, or specific fields from log entries (count for error frequency, *_all for batch extraction)
• Configuration files: Parse simple key-value pairs or extract section content (after_all for all values, count for validation)
• HTML/XML processing: Extract content from known, simple tag structures (between_all for multiple tags, *_at for specific positions)
• URL manipulation: Extract basic domains, paths, or query parameters (count for segment counting, before_at/after_at for path navigation)
• CSV/TSV processing: Navigate simple columnar data (*_at for specific columns, count for field validation)
• Template processing: Extract and count basic placeholders (between_all for all variables, count for validation)

Scaling characteristics:

• Large files: Basic functions (before, after, between) scale linearly
• Multiple extractions: *_all functions have O(m²) component but m is typically small
• Memory constrained environments: Use basic functions when possible; *_all functions require more memory
• Batch processing: *_all functions more efficient than repeated individual calls

Function Selection Guidelines:

• Single extraction: Use before, after, between for best performance
• Specific position: Use *_at functions when you know the index
• Multiple results: Use *_all functions for batch extraction
• Counting only: Use count - most memory efficient for frequency analysis
• Large strings with many patterns: Consider memory usage of *_all functions

Comparison with alternatives:

• vs. Regular expressions: May be faster for simple pattern matching due to no regex compilation step
• vs. Manual string iteration: Comparable performance with built-in error handling and cleaner syntax
• vs. Split-based approaches: Basic functions may be more efficient (stop at first match); *_all functions use full split but avoid repeated parsing
• vs. Multiple individual calls: *_all functions may be more efficient than repeated calls for batch extractions

Optimization Tips

1. Pattern placement: Consider placing the most unique part of your pattern first (may improve performance in some cases)
2. Function selection:
   • Use count instead of length(before_all(...)) for counting
   • Use *_at when you know the specific index needed
   • Use *_all for batch operations instead of multiple individual calls
3. For between() operations: More unique start patterns improve performance
4. Memory considerations:
   • Basic functions have lower memory overhead
   • *_all functions create intermediate lists - consider this for large datasets
   • count uses less memory when you only need frequency information
5. Pattern considerations: Shorter, more specific patterns can reduce false matches

Development

gleam test  # Run the tests
gleam format # Format the code

Contributing

Contributions are welcome! Please feel free to submit a Pull Request!

Documentation

Further documentation can be found at https://hexdocs.pm/string_editor.