In certain cases, cant use '_' at new_post_name.

[tetsuyainfra]

Check List

• I have already read Docs page & Troubleshooting page
• I have already searched existing issues and they are not help to me
• I examined error or warning messages and it's difficult to solve
• Using the latest version of Hexo (run hexo version to check)
• Node.js is higher than 8.6.0

Expected behavior

`INFO Created: ~/work/jisaku-club/blog/source/_posts/2019-11-19_test.md

Actual behavior`

INFO Created: ~/work/jisaku-club/blog/source/_posts/2019-11-undefinedtest.md

How to reproduce?

• Change _config.yaml as bellow
• hexo new post test

Environment & Settings

Node.js & npm version

node v12.13.0
npm 6.13.0

Your site _config.yml (Optional)

new_post_name: ':year-:month-:day_:title.md' # File name of new posts

Hexo and Plugin version(npm ls --depth 0)

hexo-site@0.0.0 /home/tetsuyainfra/work/jisaku-club/blog
├── hexo@4.0.0
├── hexo-deployer-git@2.0.0
├── hexo-generator-archive@1.0.0
├── hexo-generator-category@1.0.0
├── hexo-generator-index@1.0.0
├── hexo-generator-tag@1.0.0
├── hexo-renderer-ejs@1.0.0
├── hexo-renderer-marked@2.0.0
├── hexo-renderer-stylus@1.1.0
└── hexo-server@1.0.0

Your package.json package.json

{                                                 
  "name": "hexo-site",                            
  "version": "0.0.0",                             
  "private": true,                                
  "scripts": {                                    
    "build": "hexo generate",                     
    "clean": "hexo clean",                        
    "deploy": "hexo deploy",                      
    "server": "hexo server"                       
  },                                              
  "hexo": {                                       
    "version": "4.0.0"                            
  },                                              
  "dependencies": {                               
    "hexo": "^4.0.0",                             
    "hexo-deployer-git": "^2.0.0",                
    "hexo-generator-archive": "^1.0.0",           
    "hexo-generator-category": "^1.0.0",          
    "hexo-generator-index": "^1.0.0",             
    "hexo-generator-tag": "^1.0.0",               
    "hexo-renderer-ejs": "^1.0.0",                
    "hexo-renderer-marked": "^2.0.0",             
    "hexo-renderer-stylus": "^1.1.0",             
    "hexo-server": "^1.0.0"                       
  }                                               
}                                                 

Others

When I followed the code, I found out that I could change regex key will be ok.
https://github.com/hexojs/hexo-util/blob/master/lib/permalink.js#L5

But present code, variable keys already include '_'.

hexo/lib/plugins/filter/new_post_path.js

Lines 10 to 17 in eb33af0

	const reservedKeys = {
	year: true,
	month: true,
	i_month: true,
	day: true,
	i_day: true,
	title: true
	};

I thought I could use the current key to regular expression,
but it matter if the number of keys increases in the future.

I couldn't come up with a way to solve it.

[seaoak]

@tetsuyainfra 's comment is right.
I think this behavior is a bug of hexo-util/lib/permalink.js.

I make a new patch hexojs/hexo-util#139.

[curbengh]

Fix has been merged, please wait for its release in hexo-util 1.6.0.