Regular Expressions

The regexp package matches text patterns using Go’s RE2-based regular-expression engine.

regexp.MustCompile constructs a reusable pattern and panics if the pattern is invalid; use regexp.Compile when handling a pattern supplied by a user or another fallible source. MatchString checks for a match, and FindAllString extracts matching parts.

Go regexp supports a deliberately bounded feature set and does not offer every construct found in Perl-compatible engines, such as backreferences. Prefer simpler string operations when a regex adds no value.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import (
	"fmt"
	"regexp"
)

func main() {
	pattern := regexp.MustCompile(`[0-9]+`)
	text := "Boxes: 12, 7 and 35"
	fmt.Println("Numbers:", pattern.FindAllString(text, -1))
	fmt.Println("Contains number:", pattern.MatchString(text))
}

Output

Numbers: [12 7 35]
Contains number: true

The pattern matches one or more ASCII digits. FindAllString extracts the three number substrings in text order, while MatchString reports true.

• Backreferences such as \1 are not supported by Go regexp.
• A regex that matches anywhere is different from one anchored with ^ and $.

Write a pattern that matches a word starting with capital G, then test it with MatchString.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Regular Expressions in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.