Using package`regexp` to find all mactch substring in Golang, but get unexpected result

Question

I am using package regexp to find all mactch substring in Golang, but get unexpected result.Here is my code:

package main

import (
    "fmt"
    "regexp"
)

func main() {
    str := "build: xxxxxx Prefix:middle#6\nPrefix:middle#16026Prefix:middle#1111\n Prefix:middle#110 Prefix:middle.#2 Prefix:middl.e#111 Prefix:middle#112"
    regexpStr := "\\bPrefix:([a-zA-Z0-9]+[\\w-.]+[^.])#[0-9]+"
    re := regexp.MustCompile(regexpStr)
    matchs := re.FindAllString(str, -1)
    fmt.Println(matchs)
}

You can see it in https://go.dev/play/p/XFSMW09MKxV.

expected:

[Prefix:middle#6 Prefix:middle#110 Prefix:middl.e#111 Prefix:middle#112]

But I got:

[Prefix:middle#6 Prefix:middle#16026 Prefix:middle#110 Prefix:middl.e#111 Prefix:middle#112]

Why Prefix:middle#16026 macthed? Could someone tell me the reason? And how to fix it, thx.

Here is the rules for what should match:

I want to extract Prefix:${middle}#${number} in a String.

${middle} rules:
- Allowed characters: letters, numbers, underscores, underscores, dots
- Must begin with a letter or number
- Can't end with a dot
${number} rules:
- Shoule be number
Prefix:${middle}#${number} can appear at the beginning or end of a string, or in the middle of a string, but：
- Appear at the beginning of the string, it needs to be followed by a space or /n;
- Appear at the end of the string, it needs to be preceded by a space or /n;
- in the middle of the string, but it needs to be preceded and followed by a newline symbol or a space.

What I expect is that [Prefix:middle#6 Prefix:middle#110 Prefix:middl.e#111 Prefix:middle#112]，How can I fix it? What kind of regexpStr should I use?@Biffen — spike014
– spike014, Commented Sep 16, 2022 at 8:47
Could you explain the rules (in English if not in regex) for what should match? The current pattern matches Prefix:middle#16026 and I can’t see a reason, or even an intention, for why it shouldn’t. — Biffen
– Biffen, Commented Sep 16, 2022 at 8:49
@Biffen I update the question to add the rules. Cound you pls read it? — spike014
– spike014, Commented Sep 16, 2022 at 9:15

Wiktor Stribiżew · Accepted Answer · 2022-09-19 13:53:42Z

2

You can use the following regex with regexp.FindAllStringSubmatch:

(?:\s|^)(Prefix:[a-zA-Z0-9][\w.-]*[^.]#\d+)(?:\s|$)

See the regex demo.

Note that this pattern will only work after doubling whitespaces in the string because both the whitespace boundaries, (?:\s|^) and (?:\s|$), are consuming patterns, and will prevent getting consecutive matches. Hence, regexp.MustCompile(\s).ReplaceAllString(str, "$0$0") or similar should be used before running the above regex.

Details:

(?:\s|^) - either a whitespace or start of string
(Prefix:[a-zA-Z0-9][\w.-]*[^.]#\d+) - Group 1:
- Prefix: - a fixed string
- [a-zA-Z0-9] - an alphanumeric
- [\w.-]* - zero or more letters, digits, underscores, dots or hyphens
- [^.] - a char other than .
- # - a # char
- \d+ - one or more digits
(?:\s|$) - either a whitespace or end of string

See the Go demo:

package main

import (
    "fmt"
    "regexp"
)

func main() {
    str := "Prefix:middle#113 build: xxxxxx Prefix:middle#6\nPrefix:middle#16026Prefix:middle#1111\n Prefix:middle#110 Prefix:middle.#2 Prefix:middl.e#111 Prefix:middle#112"
    re := regexp.MustCompile(`(?:\s|^)(Prefix:[a-zA-Z0-9][\w.-]*[^.]#\d+)(?:\s|$)`)
    matchs := re.FindAllStringSubmatch(regexp.MustCompile(`\s`).ReplaceAllString(str, "$0$0"), -1)
    for _, m := range matchs {
        fmt.Println(m[1])
    }
}

Output:

Prefix:middle#113
Prefix:middle#6
Prefix:middle#110
Prefix:middl.e#111
Prefix:middle#112

edited Sep 19, 2022 at 13:53

answered Sep 16, 2022 at 8:49

Wiktor Stribiżew

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12 Comments

spike014 Over a year ago

Thx for response quickly! I update the question to add the rules. Cound you pls read it again?

Wiktor Stribiżew Over a year ago

@spike014 What does not work? What do you expect?

spike014 Over a year ago

I comment in the end of the code, go.dev/play/p/lPZAjMyGIHH

Wiktor Stribiżew Over a year ago

@spike014 See the update.

Wiktor Stribiżew Over a year ago

@spike014 See this Go demo, does it work as expected?

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