CODEWITHSUNDEEP
stringgo
Kshitij Saraogi
Kshitij Saraogi

Reputation: 7609

Parsing formatted strings in Go

The Problem

I have slice of string values wherein each value is formatted based on a template. In my particular case, I am trying to parse Markdown URLs as shown below:

- [What did I just commit?](#what-did-i-just-commit)
- [I wrote the wrong thing in a commit message](#i-wrote-the-wrong-thing-in-a-commit-message)
- [I committed with the wrong name and email configured](#i-committed-with-the-wrong-name-and-email-configured)
- [I want to remove a file from the previous commit](#i-want-to-remove-a-file-from-the-previous-commit)
- [I want to delete or remove my last commit](#i-want-to-delete-or-remove-my-last-commit)
- [Delete/remove arbitrary commit](#deleteremove-arbitrary-commit)
- [I tried to push my amended commit to a remote, but I got an error message](#i-tried-to-push-my-amended-commit-to-a-remote-but-i-got-an-error-message)
- [I accidentally did a hard reset, and I want my changes back](#i-accidentally-did-a-hard-reset-and-i-want-my-changes-back)

What I want to do?

I am looking for ways to parse this into a value of type:

type Entity struct {
    Statement string
    URL string
}

What have I tried?

As you can see, all the items follow the pattern: - [{{ .Statement }}]({{ .URL }}). I tried using the fmt.Sscanf function to scan each string as:

var statement, url string
fmt.Sscanf(s, "[%s](%s)", &statement, &url)

This results in:

statement = "I"
url = ""

The issue is with the scanner storing space-separated values only. I do not understand why the URL field is not getting populated based on this rule.

How can I get the Markdown values as mentioned above?

EDIT: As suggested by Marc, I will add couple of clarification points:

  1. This is a general purpose question on parsing strings based on a format. In my particular case, a Markdown parser might help me but my intention to learn how to handle such cases in general where a library might not exist.
  2. I have read the official documentation before posting here.

Upvotes: 0

Views: 2435

Answers (2)

benschumacher
benschumacher

Reputation: 162

You could create a simple lexer in pure-Go code for this use case. There's a great talk by Rob Pike from years ago that goes into the design of text/template which would be applicable. The implementation chains together a series of state functions into an overall state machine, and delivers the tokens out through a channel (via Goroutine) for later processing.

Upvotes: 0

icza
icza

Reputation: 417612

Note: The following solution only works for "simple", non-escaped input markdown links. If this suits your needs, go ahead and use it. For full markdown-compatibility you should use a proper markdown parser such as gopkg.in/russross/blackfriday.v2.

You could use regexp to get the link text and the URL out of a markdown link.

So the general input text is in the form of:

[some text](somelink)

A regular expression that models this:

\[([^\]]+)\]\(([^)]+)\)

Where:

  • \[ is the literal [
  • ([^\]]+) is for the "some text", it's everything except the closing square brackets
  • \] is the literal ]
  • \( is the literal (
  • ([^)]+) is for the "somelink", it's everything except the closing brackets
  • \) is the literal )

Example:

r := regexp.MustCompile(`\[([^\]]+)\]\(([^)]+)\)`)

inputs := []string{
    "[Some text](#some/link)",
    "[What did I just commit?](#what-did-i-just-commit)",
    "invalid",
}

for _, input := range inputs {
    fmt.Println("Parsing:", input)
    allSubmatches := r.FindAllStringSubmatch(input, -1)

    if len(allSubmatches) == 0 {
        fmt.Println("   No match!")
    } else {
        parts := allSubmatches[0]
        fmt.Println("   Text:", parts[1])
        fmt.Println("   URL: ", parts[2])
    }
}

Output (try it on the Go Playground):

Parsing: [Some text](#some/link)
   Text: Some text
   URL:  #some/link
Parsing: [What did I just commit?](#what-did-i-just-commit)
   Text: What did I just commit?
   URL:  #what-did-i-just-commit
Parsing: invalid
   No match!

Upvotes: 3

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