String Operators - Rule Engine JS

Overview

String operators enable powerful text matching and validation capabilities. Rule Engine JS provides four string operators for different text comparison needs:

contains

Check if string contains substring

startsWith

Check if string starts with prefix

endsWith

Check if string ends with suffix

regex

Match against regular expression patterns

Architecture

String operators are implemented across two classes extending BaseOperator: Source Files:

String operators: src/operators/string.js
Regex operator: src/operators/regex.js
Base operator: src/operators/base/BaseOperator.js
Type utilities: src/utils/TypeUtils.js
Unit tests: tests/unit/operators/string.test.js

Key Features

Dynamic field comparison - Compare two fields or field to literal value
Case sensitive by default - Exact matching for security
Automatic type coercion - Non-string values converted to strings
Pattern caching - Regex patterns cached for performance
Comprehensive validation - Validates argument count and types

`contains` - Substring Match

Check if a string contains a substring.

Syntax

{ contains: [haystack, needle] }
{ contains: [haystack, needle, options] }

Parameters

haystack

string

required

The string to search in - field path or literal value

needle

string

required

The substring to search for - field path or literal value

options

object

Configuration options

Show properties

strict

boolean

default:"false"

Enable strict type checking - prevents automatic string coercion

Returns

boolean - true if haystack contains needle, false otherwise

Examples

Basic Contains
Dynamic Field Comparison
Email Domain Validation
With Rule Helpers

import { createRuleEngine } from 'rule-engine-js';

const engine = createRuleEngine();
const data = {
  user: {
    email: 'john@company.com',
    bio: 'Software Engineer at TechCorp',
  }
};

// Check if email contains domain
engine.evaluateExpr({ contains: ['user.email', '@company.com'] }, data);
// Result: { success: true }

// Check bio for keyword
engine.evaluateExpr({ contains: ['user.bio', 'Software'] }, data);
// Result: { success: true }

// Case sensitive - will fail
engine.evaluateExpr({ contains: ['user.bio', 'software'] }, data);
// Result: { success: false }

Common Use Cases

Search Functionality

const article = {
  title: 'Introduction to React Hooks',
  content: 'React Hooks provide a way to use state...',
  tags: ['react', 'hooks', 'javascript']
};

// Search in title or content
const searchRule = {
  or: [
    { contains: ['title', 'React'] },
    { contains: ['content', 'React'] }
  ]
};

engine.evaluateExpr(searchRule, article);
// Result: { success: true }

Content Filtering

const post = {
  content: 'Check out this amazing product!',
  author: 'spammer@example.com'
};

// Filter spam content
const spamRule = {
  or: [
    { contains: ['content', 'amazing product'] },
    { contains: ['content', 'click here'] },
    { contains: ['author', 'spammer'] }
  ]
};

engine.evaluateExpr(spamRule, post);
// Result: { success: true } - Flagged as spam

Category Matching

const product = {
  name: 'MacBook Pro 16-inch',
  category: 'Electronics > Computers > Laptops'
};

// Check product category
const isLaptop = { contains: ['category', 'Laptops'] };
const isElectronics = { contains: ['category', 'Electronics'] };

engine.evaluateExpr(isLaptop, product);
// Result: { success: true }

Case Sensitivity: contains is case-sensitive by default. “Software” !== “software”

`startsWith` - Prefix Match

Check if a string starts with a specific prefix.

Syntax

{ startsWith: [string, prefix] }
{ startsWith: [string, prefix, options] }

Parameters

string

required

The string to check - field path or literal value

prefix

string

required

The prefix to match - field path or literal value

options

object

Configuration options

Show properties

strict

boolean

default:"false"

Enable strict type checking

Returns

boolean - true if string starts with prefix, false otherwise

Examples

Basic StartsWith
Dynamic Prefix
ID Prefix Validation

const data = {
  user: {
    name: 'John Doe',
    username: 'john_doe_123'
  },
  url: 'https://example.com/page'
};

// Check name prefix
engine.evaluateExpr({ startsWith: ['user.name', 'John'] }, data);
// Result: { success: true }

// Check URL protocol
engine.evaluateExpr({ startsWith: ['url', 'https'] }, data);
// Result: { success: true }

// Check username pattern
engine.evaluateExpr({ startsWith: ['user.username', 'john_'] }, data);
// Result: { success: true }

Common Use Cases

URL Protocol Validation

const links = [
  { url: 'https://secure.example.com' },
  { url: 'http://example.com' },
  { url: 'ftp://files.example.com' }
];

// Only allow HTTPS
const httpsOnly = { startsWith: ['url', 'https://'] };

links.forEach(link => {
  const result = engine.evaluateExpr(httpsOnly, link);
  console.log(`${link.url}: ${result.success}`);
});

File Path Validation

const file = {
  path: '/app/uploads/images/photo.jpg',
  allowedPrefix: '/app/uploads/'
};

// Ensure file is in allowed directory
const inAllowedDir = {
  startsWith: ['path', 'allowedPrefix']
};

engine.evaluateExpr(inAllowedDir, file);
// Result: { success: true }

Phone Number Format

const contact = {
  phone: '+1-555-123-4567',
  country: 'US'
};

// Validate US phone number format
const usPhoneRule = { startsWith: ['phone', '+1-'] };

engine.evaluateExpr(usPhoneRule, contact);
// Result: { success: true }

`endsWith` - Suffix Match

Check if a string ends with a specific suffix.

Syntax

{ endsWith: [string, suffix] }
{ endsWith: [string, suffix, options] }

Parameters

string

required

The string to check - field path or literal value

suffix

string

required

The suffix to match - field path or literal value

options

object

Configuration options

Show properties

strict

boolean

default:"false"

Enable strict type checking

Returns

boolean - true if string ends with suffix, false otherwise

Examples

Basic EndsWith
Dynamic Suffix
File Type Validation

const data = {
  user: {
    email: 'john@company.com',
  },
  file: {
    name: 'document.pdf',
    backup: 'document.pdf.bak'
  }
};

// Check email domain
engine.evaluateExpr({ endsWith: ['user.email', '.com'] }, data);
// Result: { success: true }

// Check file extension
engine.evaluateExpr({ endsWith: ['file.name', '.pdf'] }, data);
// Result: { success: true }

// Check backup file
engine.evaluateExpr({ endsWith: ['file.backup', '.bak'] }, data);
// Result: { success: true }

Common Use Cases

File Upload Validation

const upload = {
  filename: 'profile-photo.jpg',
  allowedTypes: ['.jpg', '.jpeg', '.png', '.gif']
};

// Accept only image files
const imageRule = {
  or: [
    { endsWith: ['filename', '.jpg'] },
    { endsWith: ['filename', '.jpeg'] },
    { endsWith: ['filename', '.png'] },
    { endsWith: ['filename', '.gif'] }
  ]
};

engine.evaluateExpr(imageRule, upload);
// Result: { success: true }

Email Domain Restriction

const user = {
  email: 'employee@company.com',
  isInternal: true
};

// Only allow company email
const companyEmailRule = {
  and: [
    { endsWith: ['email', '@company.com'] },
    { eq: ['isInternal', true] }
  ]
};

engine.evaluateExpr(companyEmailRule, user);
// Result: { success: true }

URL Path Matching

const request = {
  path: '/api/v1/users/profile.json',
  requiresJson: true
};

// API must return JSON
const jsonApiRule = {
  and: [
    { startsWith: ['path', '/api/'] },
    { endsWith: ['path', '.json'] }
  ]
};

engine.evaluateExpr(jsonApiRule, request);
// Result: { success: true }

`regex` - Regular Expression Match

Match strings against regular expression patterns with support for flags and pattern caching.

Syntax

{ regex: [text, pattern] }
{ regex: [text, pattern, options] }

Parameters

text

string

required

The text to match against - field path or literal value

pattern

string

required

The regular expression pattern - field path or literal value

options

object

Configuration options

Show properties

flags

string

default:""

Regex flags: i (case-insensitive), g (global), m (multiline), etc.

strict

boolean

default:"false"

Enable strict type checking

Returns

boolean - true if text matches pattern, false otherwise

Examples

Email Validation
Phone Number Validation
Case-Insensitive Search
Dynamic Patterns
Password Strength

const user = {
  email: 'john.doe@company.com'
};

// Validate email format
const emailRule = {
  regex: ['email', '^[\\w\\.-]+@[\\w\\.-]+\\.[a-zA-Z]{2,}$']
};

engine.evaluateExpr(emailRule, user);
// Result: { success: true }

// Test with invalid email
const invalidUser = { email: 'invalid.email' };
engine.evaluateExpr(emailRule, invalidUser);
// Result: { success: false }

Common Use Cases

URL Validation

const link = {
  url: 'https://www.example.com/path?query=value'
};

// Validate URL format
const urlRule = {
  regex: ['url', '^https?:\\/\\/[\\w\\.-]+(:[\\d]+)?(\\/[\\w\\.-]*)*\\??([\\w=&]*)$']
};

engine.evaluateExpr(urlRule, link);
// Result: { success: true }

Credit Card Validation

const payment = {
  cardNumber: '4532-1234-5678-9010'
};

// Validate credit card format (Visa)
const visaRule = {
  regex: ['cardNumber', '^4[0-9]{3}-?[0-9]{4}-?[0-9]{4}-?[0-9]{4}$']
};

engine.evaluateExpr(visaRule, payment);
// Result: { success: true }

Date Format Validation

const event = {
  date: '2024-12-25',
  time: '14:30:00'
};

// Validate ISO date format
const dateRule = {
  regex: ['date', '^\\d{4}-\\d{2}-\\d{2}$']
};

// Validate time format
const timeRule = {
  regex: ['time', '^([01]\\d|2[0-3]):[0-5]\\d:[0-5]\\d$']
};

engine.evaluateExpr({ and: [dateRule, timeRule] }, event);
// Result: { success: true }

Username Validation

const user = {
  username: 'john_doe_123'
};

// Username: 3-20 chars, alphanumeric + underscore
const usernameRule = {
  regex: ['username', '^[a-zA-Z0-9_]{3,20}$']
};

engine.evaluateExpr(usernameRule, user);
// Result: { success: true }

Performance: Regex patterns are cached automatically for better performance when evaluating the same pattern multiple times.

Error Handling

Common Errors

Non-String Operands

const data = { user: { age: 28 } };

// Trying to use contains on number
const result = engine.evaluateExpr(
  { contains: ['user.age', '28'] },
  data
);

// Returns:
// {
//   success: false,
//   error: "CONTAINS operator requires string operands"
// }

Invalid Regex Pattern

const data = { text: 'hello' };

// Invalid regex syntax
const result = engine.evaluateExpr(
  { regex: ['text', '[invalid('] },
  data
);

// Returns:
// {
//   success: false,
//   error: "Invalid regex pattern: [invalid("
// }

Missing Arguments

// Missing second argument
const result = engine.evaluateExpr(
  { contains: ['user.email'] },
  data
);

// Returns:
// {
//   success: false,
//   error: "CONTAINS operator requires 2-3 arguments, got 1"
// }

Undefined Field Access

const data = { user: { name: 'John' } };

// Field doesn't exist - undefined coerced to empty string
const result = engine.evaluateExpr(
  { contains: ['user.email', '@'] },
  data
);

// Returns:
// {
//   success: false,
//   error: "CONTAINS operator requires string operands"
// }

Error Recovery

function safeStringMatch(engine, operator, field, pattern, data, fallback = false) {
  const result = engine.evaluateExpr({ [operator]: [field, pattern] }, data);

  if (!result.success) {
    console.error(`String match failed: ${result.error}`);
    return fallback;
  }

  return result.success;
}

// Usage
const hasKeyword = safeStringMatch(
  engine,
  'contains',
  'content',
  'important',
  article,
  false
);

Type Coercion

String operators support automatic type coercion in loose mode:

Input Type	Coercion Behavior	Example
`string`	No change	`"hello"` → `"hello"`
`number`	Converted to string	`123` → `"123"`
`boolean`	Converted to string	`true` → `"true"`
`null`	Error thrown	`null` → Error
`undefined`	Error thrown	`undefined` → Error
`object`	Error thrown	`{}` → Error

Strict Mode

const data = { value: 123 };

// Loose mode (default) - coerces number to string
engine.evaluateExpr({ contains: ['value', '12'] }, data);
// Result: { success: true } - "123" contains "12"

// Strict mode - rejects non-string
const strictEngine = createRuleEngine({ strict: true });
strictEngine.evaluateExpr({ contains: ['value', '12'] }, data);
// Result: { success: false, error: "requires string operands" }

Comparison Operators

eq, neq for exact matching

Array Operators

in, notIn for array membership

Special Operators

isNull, isNotNull, between

Logical Operators

and, or, not for combining rules

Numeric Operators

Numeric operations

All Operators

Complete operator reference

API Reference

For complete API documentation:

Get Started

Core Concepts

Operators

Guides

Contributing

​Overview

contains

startsWith

endsWith

regex

​Architecture

​Key Features

​contains - Substring Match

​Syntax

​Parameters

​Returns

​Examples

​Common Use Cases

​startsWith - Prefix Match

​Syntax

​Parameters

​Returns

​Examples

​Common Use Cases

​endsWith - Suffix Match

​Syntax

​Parameters

​Returns

​Examples

​Common Use Cases

​regex - Regular Expression Match

​Syntax

​Parameters

​Returns

​Examples

​Common Use Cases

​Error Handling

​Common Errors

​Error Recovery

​Type Coercion

​Strict Mode

​Related Operators

Comparison Operators

Array Operators

Special Operators

Logical Operators

Numeric Operators

All Operators

​API Reference

Overview

Architecture

Key Features

`contains` - Substring Match

Syntax

Parameters

Returns

Examples

Common Use Cases

`startsWith` - Prefix Match

Syntax

Parameters

Returns

Examples

Common Use Cases

`endsWith` - Suffix Match

Syntax

Parameters

Returns

Examples

Common Use Cases

`regex` - Regular Expression Match

Syntax

Parameters

Returns

Examples

Common Use Cases

Error Handling

Common Errors

Error Recovery

Type Coercion

Strict Mode

Related Operators

API Reference