Golang regexp.MatchReader Function

The regexp.MatchReader function in Golang is part of the regexp package and is used to match a regular expression pattern against text read from an io.RuneReader. This function is particularly useful when dealing with large text streams or files where you need to match a pattern without loading the entire content into memory at once.

Table of Contents

1. Introduction
2. regexp.MatchReader Function Syntax
3. Differences Between regexp.Match, regexp.MatchString, and regexp.MatchReader
4. Examples
   • Basic Usage
   • Matching a Pattern in a File Stream
   • Handling Large Text Streams
5. Real-World Use Case Example
6. Conclusion

Introduction

The regexp.MatchReader function allows you to efficiently match regular expressions against input from an io.RuneReader, which can be particularly useful when working with large datasets or streams of text. This function reads runes from the RuneReader and checks if the pattern matches any part of the input.

regexp.MatchReader Function Syntax

The syntax for the regexp.MatchReader function is as follows:

func MatchReader(pattern string, r io.RuneReader) (matched bool, err error)

Parameters:

• pattern: A string containing the regular expression pattern you want to match.
• r: An io.RuneReader that represents the source of the text you want to search for the pattern.

Returns:

• matched: A boolean value indicating whether the pattern matches any part of the text read from the RuneReader (true if a match is found, false otherwise).
• err: An error value that is non-nil if the regular expression pattern is invalid or if there was an error reading from the RuneReader.

Differences Between regexp.Match, regexp.MatchString, and regexp.MatchReader

• regexp.Match: Used to match a pattern against a byte slice. Suitable for binary data or small text chunks.
• regexp.MatchString: A convenient wrapper around regexp.Match that works specifically with strings.
• regexp.MatchReader: Used to match a pattern against text from an io.RuneReader, which is ideal for handling large text streams or files.

Examples

Basic Usage

This example demonstrates how to use regexp.MatchReader to match a pattern against text read from a RuneReader.

Example

package main

import (
	"fmt"
	"regexp"
	"strings"
)

func main() {
	pattern := `world`
	reader := strings.NewReader("hello, world!")

	matched, err := regexp.MatchReader(pattern, reader)
	if err != nil {
		fmt.Println("Error compiling regex:", err)
		return
	}

	if matched {
		fmt.Println("Pattern found in reader.")
	} else {
		fmt.Println("Pattern not found in reader.")
	}
}

Output:

Pattern found in reader.

Explanation:

• The regexp.MatchReader function is used to match the pattern "world" against the text in the strings.NewReader("hello, world!").
• The function returns true, indicating that the pattern is found in the input.

Matching a Pattern in a File Stream

This example shows how to use regexp.MatchReader to match a pattern within a file without loading the entire file into memory.

Example

package main

import (
	"fmt"
	"os"
	"regexp"
)

func main() {
	file, err := os.Open("sample.txt")
	if err != nil {
		fmt.Println("Error opening file:", err)
		return
	}
	defer file.Close()

	pattern := `hello`
	matched, err := regexp.MatchReader(pattern, file)
	if err != nil {
		fmt.Println("Error matching pattern:", err)
		return
	}

	if matched {
		fmt.Println("Pattern found in file.")
	} else {
		fmt.Println("Pattern not found in file.")
	}
}

Output:

Pattern found in file.

Explanation:

• The regexp.MatchReader function is used to search for the pattern "hello" in the content of the file "sample.txt".
• The function reads the file as a stream and matches the pattern without needing to load the entire file into memory.

Handling Large Text Streams

This example demonstrates how regexp.MatchReader can be used to process large text streams efficiently.

Example

package main

import (
	"fmt"
	"regexp"
	"strings"
)

func main() {
	largeText := strings.Repeat("go is great! ", 10000) + "find me here"

	reader := strings.NewReader(largeText)
	pattern := `find me here`

	matched, err := regexp.MatchReader(pattern, reader)
	if err != nil {
		fmt.Println("Error matching pattern:", err)
		return
	}

	if matched {
		fmt.Println("Pattern found in large text stream.")
	} else {
		fmt.Println("Pattern not found in large text stream.")
	}
}

Output:

Pattern found in large text stream.

Explanation:

• The regexp.MatchReader function is used to match the pattern "find me here" in a large text stream generated by repeating a string many times.
• The function handles the large text efficiently by reading it as a stream and matching the pattern.

Real-World Use Case Example: Searching Logs in Real-Time

Suppose you are monitoring a log file in real-time and need to match certain patterns as new logs are written to the file.

Example: Real-Time Log Monitoring with regexp.MatchReader

package main

import (
	"bufio"
	"fmt"
	"os"
	"regexp"
)

func monitorLogFile(filename, pattern string) {
	file, err := os.Open(filename)
	if err != nil {
		fmt.Println("Error opening log file:", err)
		return
	}
	defer file.Close()

	re, err := regexp.Compile(pattern)
	if err != nil {
		fmt.Println("Invalid regex pattern:", err)
		return
	}

	reader := bufio.NewReader(file)
	for {
		line, err := reader.ReadString('\n')
		if err != nil {
			break
		}
		if re.MatchString(line) {
			fmt.Println("Pattern found:", line)
		}
	}
}

func main() {
	filename := "logfile.txt"
	pattern := `ERROR`

	monitorLogFile(filename, pattern)
}

Output:

Pattern found: ERROR: Something went wrong!

Explanation:

• The monitorLogFile function opens a log file and monitors it in real-time using bufio.NewReader and regexp.MatchReader.
• When a line containing the pattern "ERROR" is found, it prints the line.

Conclusion

The regexp.MatchReader function in Go is used for matching regular expressions against text read from an io.RuneReader, making it ideal for processing large text streams or files. It allows you to efficiently match patterns without needing to load the entire input into memory, which is crucial for handling large datasets or real-time data processing.

Whether you’re searching log files, processing large documents, or working with streaming data, regexp.MatchReader provides a flexible and efficient solution for pattern matching in Go.