# Regular Expressions

This Go code demonstrates the usage of the `regexp` package for regular expression matching and manipulation. Let's go through the code with inline comments and explanations:

```go
// Importing necessary packages.
import (
	"bytes"
	"fmt"
	"regexp"
)

// The main function, where the execution of the program begins.
func main() {
	// Using MatchString to check if a string matches a regular expression pattern.
	match, _ := regexp.MatchString("p([a-z]+)ch", "peach")
	fmt.Println(match)

	// Compiling a regular expression pattern for reuse.
	r, _ := regexp.Compile("p([a-z]+)ch")

	// Using MatchString with the compiled regex.
	fmt.Println(r.MatchString("peach"))

	// Finding the first match in the input string.
	fmt.Println(r.FindString("peach punch"))

	// Finding the start and end indices of the first match.
	fmt.Println("idx:", r.FindStringIndex("peach punch"))

	// Finding submatches of the first match.
	fmt.Println(r.FindStringSubmatch("peach punch"))

	// Finding start and end indices of submatches of the first match.
	fmt.Println(r.FindStringSubmatchIndex("peach punch"))

	// Finding all matches in the input string.
	fmt.Println(r.FindAllString("peach punch pinch", -1))

	// Finding all start and end indices of submatches in all matches.
	fmt.Println("all:", r.FindAllStringSubmatchIndex(
		"peach punch pinch", -1))

	// Finding a specific number of matches.
	fmt.Println(r.FindAllString("peach punch pinch", 2))

	// Matching using a byte slice.
	fmt.Println(r.Match([]byte("peach")))

	// Compiling a regular expression using MustCompile for simplicity.
	r = regexp.MustCompile("p([a-z]+)ch")
	fmt.Println("regexp:", r)

	// Replacing all matches with a specified string.
	fmt.Println(r.ReplaceAllString("a peach", "<fruit>"))

	// Replacing all matches using a function.
	in := []byte("a peach")
	out := r.ReplaceAllFunc(in, bytes.ToUpper)
	fmt.Println(string(out))
}
```

#### Output

```
true
true
peach
idx: [0 5]
[peach ea]
[0 5 1 3]
[peach punch pinch]
all: [[0 5 1 3] [6 11 7 9] [12 17 13 15]]
[peach punch]
true
regexp: p([a-z]+)ch
a <fruit>
a PEACH
```

Explanation:

1. **`MatchString` and `Compile`:**
   * `MatchString` checks if a string matches a regular expression pattern.
   * `Compile` is used to compile a regular expression pattern for later use.
2. **Finding Matches:**
   * `FindString` finds the first match in the input string.
   * `FindStringIndex` finds the start and end indices of the first match.
   * `FindStringSubmatch` finds submatches of the first match.
   * `FindStringSubmatchIndex` finds start and end indices of submatches of the first match.
   * `FindAllString` finds all matches in the input string.
   * `FindAllStringSubmatchIndex` finds all start and end indices of submatches in all matches.
   * `FindAllString` can be used to find a specific number of matches.
3. **Matching and Replacing:**
   * `Match` is used to match using a byte slice.
   * `MustCompile` is used to simplify regular expression compilation.
   * `ReplaceAllString` replaces all matches with a specified string.
   * `ReplaceAllFunc` replaces all matches using a function (`bytes.ToUpper` in this case).

These examples demonstrate common operations with regular expressions using the `regexp` package in Go. Regular expressions provide powerful and flexible pattern matching capabilities.
