Post title start with number has prefixed underscore in post slug

[ariona]

• I confirm that this is an issue rather than a question.

Bug report

Steps to reproduce

• I prepared a reproduction repo, here is the reproduction repo:

• This issue doesn't need a reproduction repro, here is the steps to reproduce

When the post title start with a number, this slug is started with underscore. for example if i have this file 2020-05-05-5-problem-solving-for-frontend-developer.md.
The slug url will be:

http://example.com/_5-problem-solving-for-frontend-developer

What is expected?

The slug is not prefixed with underscore with post title started with number:

http://example.com/5-problem-solving-for-frontend-developer

What is actually happening?

The slug is prefixed with underscore:

http://example.com/_5-problem-solving-for-frontend-developer

Other relevant information

• @vuepress/core: 1.5.0
• @vuepress/theme-default: 1.5.0
• vuepress: ^1.5.0 => 1.5.0
• @vuepress/plugin-blog: ^1.9.2

[billyyyyy3320]

It's relevant to how VuePress itself generate permalink. I'll look into it when I'm free. In the meantime, you can transfer this issue to VuePress so that there're might be more people can help you.