Advanced tutorial on regular expressions in Go language: How to use zero-width assertions

Advanced tutorial on regular expressions in Go language: How to use zero-width assertions

Regular expressions are a powerful text matching tool that can be used to find and replace text of a specific pattern in a string. . The regular expression library in Go language provides rich features, including zero-width assertions, which are very useful in certain scenarios. This article will introduce you to how to use zero-width assertions in Go language to improve the flexibility of regular expressions.

Zero-width assertion is a special regular expression used to confirm the position of a subpattern (or substring) without consuming characters. It helps us find text that matches specific criteria without returning the text itself. In the Go language, there are four types of zero-width assertions: positive zero-width assertion, negative zero-width assertion, positive zero-width assertion non-capturing group, and negative zero-width assertion non-capturing group.

Positive Lookahead Assertion is used to find text with a specific pattern appearing behind a certain position. Its syntax is (?=...), where ... represents the pattern that needs to be matched. The following is an example:

package main

import (
    "fmt"
    "regexp"
)

func main() {
    str := "123abc456"
    pattern := `d(?=abc)`  // 匹配数字后面紧跟着"abc"的情况
    re := regexp.MustCompile(pattern)
    results := re.FindAllString(str, -1)
    fmt.Println(results)  // 输出：[1]
}

In the above example, what we want to match is the case where the number is followed by "abc", that is, the number "1". A forward zero-width assertion is used here, and the result returned after a successful match is the number "1".

Negative zero-width assertion (Negative Lookahead Assertion) is to find text that does not appear in a specific pattern after a certain position. Its syntax is (?!...), where ... represents the pattern that needs to be excluded. The following is an example:

package main

import (
    "fmt"
    "regexp"
)

func main() {
    str := "123abc456"
    pattern := `d(?!abc)`  // 匹配数字后面不跟着"abc"的情况
    re := regexp.MustCompile(pattern)
    results := re.FindAllString(str, -1)
    fmt.Println(results)  // 输出：[2 3]
}

In the above example, what we want to match is the case where the number is not followed by "abc", that is, the numbers "2" and "3". A negative zero-width assertion is used here, and the results returned after a successful match are the numbers "2" and "3".

Positive zero-width assertion non-capturing group (Positive Lookahead Non-Capturing Group) and negative zero-width assertion non-capturing group (Negative Lookahead Non-Capturing Group) are used similarly, except that syntax does not require Add ?= or ?! outside (...). Here is an example:

package main

import (
    "fmt"
    "regexp"
)

func main() {
    str := "abc123xyz"
    pattern := `(?i:[a-z]+(?=d))`  // 匹配小写字母后面紧跟着数字的情况
    re := regexp.MustCompile(pattern)
    results := re.FindAllString(str, -1)
    fmt.Println(results)  // 输出：[abc]
}

In the above example, what we want to match is the case where lowercase letters are followed by numbers, that is, "abc". A forward zero-width assertion non-capturing group is used here, and the result returned after a successful match is "abc".

Negative zero-width assertion for non-capturing groups also uses the syntax (?i:...), just add the need to exclude in (...) mode. The following is an example:

package main

import (
    "fmt"
    "regexp"
)

func main() {
    str := "abc123XYZ"
    pattern := `(?i:[a-z]+(?!123))`  // 匹配小写字母后面不跟着"123"的情况
    re := regexp.MustCompile(pattern)
    results := re.FindAllString(str, -1)
    fmt.Println(results)  // 输出：[abc]
}

In the above example, what we want to match is the case where lowercase letters are not followed by "123", that is, "abc". A negative zero-width assertion non-capturing group is used here, and the result returned after a successful match is "abc".

By using zero-width assertions, we can extend the capabilities of regular expressions to achieve more precise text matching. When using regular expressions, proper use of zero-width assertions can save code and improve matching efficiency. I hope this article will help you understand and use zero-width assertions with regular expressions in Go language.

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