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@DavidAnson @ciwchris
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Rules

This document contains a description of all rules, what they are checking for, as well as an examples of documents that break the rule and corrected versions of the examples.

MD001 - Header levels should only increment by one level at a time

Tags: headers

Aliases: header-increment

This rule is triggered when you skip header levels in a markdown document, for example:

# Header 1

### Header 3

We skipped out a 2nd level header in this document

When using multiple header levels, nested headers should increase by only one level at a time:

# Header 1

## Header 2

### Header 3

#### Header 4

## Another Header 2

### Another Header 3

MD002 - First header should be a top level header

Tags: headers

Aliases: first-header-h1

Parameters: level (number; default 1)

This rule is triggered when the first header in the document isn't a h1 header:

## This isn't a H1 header

### Another header

The first header in the document should be a h1 header:

# Start with a H1 header

## Then use a H2 for subsections

Note: The level parameter can be used to change the top level (ex: to h2) in cases where an h1 is added externally.

MD003 - Header style

Tags: headers

Aliases: header-style

Parameters: style ("consistent", "atx", "atx_closed", "setext", "setext_with_atx", "setext_with_atx_closed"; default "consistent")

This rule is triggered when different header styles (atx, setext, and 'closed' atx) are used in the same document:

# ATX style H1

## Closed ATX style H2 ##

Setext style H1
===============

Be consistent with the style of header used in a document:

# ATX style H1

## ATX style H2

The setext_with_atx and settext_with_atx_closed doc styles allow atx-style headers of level 3 or more in documents with setext style headers:

Setext style H1
===============

Setext style H2
---------------

### ATX style H3

Note: the configured header style can be a specific style to use (atx, atx_closed, setext, setext_with_atx, setext_with_atx_closed), or simply require that the usage be consistent within the document.

MD004 - Unordered list style

Tags: bullet, ul

Aliases: ul-style

Parameters: style ("consistent", "asterisk", "plus", "dash", "sublist"; default "consistent")

This rule is triggered when the symbols used in the document for unordered list items do not match the configured unordered list style:

* Item 1
+ Item 2
- Item 3

To fix this issue, use the configured style for list items throughout the document:

* Item 1
* Item 2
* Item 3

The configured list style can be a specific symbol to use (asterisk, plus, dash), can require that usage be consistent within the document, or can require that each sublist have a consistent symbol that is different from its parent list.

For example, the following is valid for the sublist style because the outer-most indent uses asterisk, the middle indent uses plus, and the inner-most indent uses dash:

* Item 1
  + Item 2
    - Item 3
  + Item 4
* Item 4
  + Item 5

MD005 - Inconsistent indentation for list items at the same level

Tags: bullet, ul, indentation

Aliases: list-indent

This rule is triggered when list items are parsed as being at the same level, but don't have the same indentation:

* Item 1
    * Nested Item 1
    * Nested Item 2
   * A misaligned item

Usually this rule will be triggered because of a typo. Correct the indentation for the list to fix it:

* Item 1
  * Nested Item 1
  * Nested Item 2
  * Nested Item 3

MD006 - Consider starting bulleted lists at the beginning of the line

Tags: bullet, ul, indentation

Aliases: ul-start-left

This rule is triggered when top level lists don't start at the beginning of a line:

Some text

  * List item
  * List item

To fix, ensure that top level list items are not indented:

Some test

* List item
* List item

Rationale: Starting lists at the beginning of the line means that nested list items can all be indented by the same amount when an editor's indent function or the tab key is used to indent. Starting a list 1 space in means that the indent of the first nested list is less than the indent of the second level (3 characters if you use 4 space tabs, or 1 character if you use 2 space tabs).

Note: This rule is triggered for the following scenario because the unordered sublist is not recognized as such by the parser. Not being nested 3 characters as required by the outer ordered list, it creates a top-level unordered list instead.

1. List item
  - List item
  - List item
1. List item

MD007 - Unordered list indentation

Tags: bullet, ul, indentation

Aliases: ul-indent

Parameters: indent (number; default 2)

This rule is triggered when list items are not indented by the configured number of spaces (default: 2).

Example:

* List item
   * Nested list item indented by 3 spaces

Corrected Example:

* List item
  * Nested list item indented by 2 spaces

Rationale (2 space indent): indenting by 2 spaces allows the content of a nested list to be in line with the start of the content of the parent list when a single space is used after the list marker.

Rationale (4 space indent): Same indent as code blocks, simpler for editors to implement. See http://www.cirosantilli.com/markdown-styleguide/#indented-lists for more information.

In addition, this is a compatibility issue with multi-markdown parsers, which require a 4 space indents. See http://support.markedapp.com/discussions/problems/21-sub-lists-not-indenting for a description of the problem.

Note: This rule applies to a sublist only if its parent lists are all also unordered (otherwise, extra indentation of ordered lists interferes with the rule).

MD009 - Trailing spaces

Tags: whitespace

Aliases: no-trailing-spaces

Parameters: br_spaces (number; default 0)

This rule is triggered on any lines that end with whitespace. To fix this, find the line that is triggered and remove any trailing spaces from the end.

The br_spaces parameter allows an exception to this rule for a specific amount of trailing spaces used to insert an explicit line break/br element. For example, set br_spaces to 2 to allow exactly 2 spaces at the end of a line.

Note: you have to set br_spaces to 2 or higher for this exception to take effect - you can't insert a br element with just a single trailing space, so if you set br_spaces to 1, the exception will be disabled, just as if it was set to the default of 0.

MD010 - Hard tabs

Tags: whitespace, hard_tab

Aliases: no-hard-tabs

Parameters: code_blocks (boolean; default true)

This rule is triggered by any lines that contain hard tab characters instead of using spaces for indentation. To fix this, replace any hard tab characters with spaces instead.

Example:

Some text

    * hard tab character used to indent the list item

Corrected example:

Some text

    * Spaces used to indent the list item instead

You have the option to exclude this rule for code blocks. To do so, set the code_blocks parameter to false. Code blocks are included by default since handling of tabs by tools is often inconsistent (ex: using 4 vs. 8 spaces).

MD011 - Reversed link syntax

Tags: links

Aliases: no-reversed-links

This rule is triggered when text that appears to be a link is encountered, but where the syntax appears to have been reversed (the [] and () are reversed):

(Incorrect link syntax)[http://www.example.com/]

To fix this, swap the [] and () around:

[Correct link syntax](http://www.example.com/)

Note: Markdown Extra-style footnotes do not trigger this rule:

For (example)[^1]

MD012 - Multiple consecutive blank lines

Tags: whitespace, blank_lines

Aliases: no-multiple-blanks

Parameters: maximum (number; default 1)

This rule is triggered when there are multiple consecutive blank lines in the document:

Some text here


Some more text here

To fix this, delete the offending lines:

Some text here

Some more text here

Note: this rule will not be triggered if there are multiple consecutive blank lines inside code blocks.

Note: The maximum parameter can be used to configure the maximum number of consecutive blank lines.

MD013 - Line length

Tags: line_length

Aliases: line-length

Parameters: line_length, code_blocks, tables (number; default 80, boolean; default true)

This rule is triggered when there are lines that are longer than the configured line length (default: 80 characters). To fix this, split the line up into multiple lines.

This rule has an exception where there is no whitespace beyond the configured line length. This allows you to still include items such as long URLs without being forced to break them in the middle.

You also have the option to exclude this rule for code blocks and tables. To do this, set the code_blocks and/or tables parameters to false.

Code blocks are included in this rule by default since it is often a requirement for document readability, and tentatively compatible with code rules. Still, some languages do not lend themselves to short lines.

MD014 - Dollar signs used before commands without showing output

Tags: code

Aliases: commands-show-output

This rule is triggered when there are code blocks showing shell commands to be typed, and the shell commands are preceded by dollar signs ($):

$ ls
$ cat foo
$ less bar

The dollar signs are unnecessary in the above situation, and should not be included:

ls
cat foo
less bar

However, an exception is made when there is a need to distinguish between typed commands and command output, as in the following example:

$ ls
foo bar
$ cat foo
Hello world
$ cat bar
baz

Rationale: it is easier to copy and paste and less noisy if the dollar signs are omitted when they are not needed. See http://www.cirosantilli.com/markdown-styleguide/#dollar-signs-in-shell-code for more information.

MD018 - No space after hash on atx style header

Tags: headers, atx, spaces

Aliases: no-missing-space-atx

This rule is triggered when spaces are missing after the hash characters in an atx style header:

#Header 1

##Header 2

To fix this, separate the header text from the hash character by a single space:

# Header 1

## Header 2

MD019 - Multiple spaces after hash on atx style header

Tags: headers, atx, spaces

Aliases: no-multiple-space-atx

This rule is triggered when more than one space is used to separate the header text from the hash characters in an atx style header:

#  Header 1

##  Header 2

To fix this, separate the header text from the hash character by a single space:

# Header 1

## Header 2

MD020 - No space inside hashes on closed atx style header

Tags: headers, atx_closed, spaces

Aliases: no-missing-space-closed-atx

This rule is triggered when spaces are missing inside the hash characters in a closed atx style header:

#Header 1#

##Header 2##

To fix this, separate the header text from the hash character by a single space:

# Header 1 #

## Header 2 ##

Note: this rule will fire if either side of the header is missing spaces.

MD021 - Multiple spaces inside hashes on closed atx style header

Tags: headers, atx_closed, spaces

Aliases: no-multiple-space-closed-atx

This rule is triggered when more than one space is used to separate the header text from the hash characters in a closed atx style header:

#  Header 1  #

##  Header 2  ##

To fix this, separate the header text from the hash character by a single space:

# Header 1 #

## Header 2 ##

Note: this rule will fire if either side of the header contains multiple spaces.

MD022 - Headers should be surrounded by blank lines

Tags: headers, blank_lines

Aliases: blanks-around-headers

This rule is triggered when headers (any style) are either not preceded or not followed by a blank line:

# Header 1
Some text

Some more text
## Header 2

To fix this, ensure that all headers have a blank line both before and after (except where the header is at the beginning or end of the document):

# Header 1

Some text

Some more text

## Header 2

Rationale: Aside from aesthetic reasons, some parsers, including kramdown, will not parse headers that don't have a blank line before, and will parse them as regular text.

MD023 - Headers must start at the beginning of the line

Tags: headers, spaces

Aliases: header-start-left

This rule is triggered when a header is indented by one or more spaces:

Some text

  # Indented header

To fix this, ensure that all headers start at the beginning of the line:

Some text

# Header

Rationale: Headers that don't start at the beginning of the line will not be parsed as headers, and will instead appear as regular text.

MD024 - Multiple headers with the same content

Tags: headers

Aliases: no-duplicate-header

This rule is triggered if there are multiple headers in the document that have the same text:

# Some text

## Some text

To fix this, ensure that the content of each header is different:

# Some text

## Some more text

Rationale: Some markdown parses generate anchors for headers based on the header name, and having headers with the same content can cause problems with this.

MD025 - Multiple top level headers in the same document

Tags: headers

Aliases: single-h1

Parameters: level (number; default 1)

This rule is triggered when a top level header is in use (the first line of the file is a h1 header), and more than one h1 header is in use in the document:

# Top level header

# Another top level header

To fix, structure your document so that there is a single h1 header that is the title for the document, and all later headers are h2 or lower level headers:

# Title

## Header

## Another header

Rationale: A top level header is a h1 on the first line of the file, and serves as the title for the document. If this convention is in use, then there can not be more than one title for the document, and the entire document should be contained within this header.

Note: The level parameter can be used to change the top level (ex: to h2) in cases where an h1 is added externally.

MD026 - Trailing punctuation in header

Tags: headers

Aliases: no-trailing-punctuation

Parameters: punctuation (string; default ".,;:!?")

This rule is triggered on any header that has a punctuation character as the last character in the line:

# This is a header.

To fix this, remove any trailing punctuation:

# This is a header

Note: The punctuation parameter can be used to specify what characters class as punctuation at the end of the header. For example, you can set it to '.,;:!' to allow headers with question marks in them, such as might be used in an FAQ.

MD027 - Multiple spaces after blockquote symbol

Tags: blockquote, whitespace, indentation

Aliases: no-multiple-space-blockquote

This rule is triggered when blockquotes have more than one space after the blockquote (>) symbol:

>  This is a block quote with bad indentation
>  there should only be one.

To fix, remove any extraneous space:

> This is a blockquote with correct
> indentation.

MD028 - Blank line inside blockquote

Tags: blockquote, whitespace

Aliases: no-blanks-blockquote

This rule is triggered when two blockquote blocks are separated by nothing except for a blank line:

> This is a blockquote
> which is immediately followed by

> this blockquote. Unfortunately
> In some parsers, these are treated as the same blockquote.

To fix this, ensure that any blockquotes that are right next to each other have some text in between:

> This is a blockquote.

And Jimmy also said:

> This too is a blockquote.

Alternatively, if they are supposed to be the same quote, then add the blockquote symbol at the beginning of the blank line:

> This is a blockquote.
>
> This is the same blockquote.

Rationale: Some markdown parsers will treat two blockquotes separated by one or more blank lines as the same blockquote, while others will treat them as separate blockquotes.

MD029 - Ordered list item prefix

Tags: ol

Aliases: ol-prefix

Parameters: style ("one", "ordered"; default "one")

This rule is triggered on ordered lists that do not either start with '1.' or do not have a prefix that increases in numerical order (depending on the configured style, which defaults to 'one').

Example valid list if the style is configured as 'one':

1. Do this.
1. Do that.
1. Done.

Example valid list if the style is configured as 'ordered':

1. Do this.
2. Do that.
3. Done.

MD030 - Spaces after list markers

Tags: ol, ul, whitespace

Aliases: list-marker-space

Parameters: ul_single, ol_single, ul_multi, ol_multi (number; default 1)

This rule checks for the number of spaces between a list marker (e.g. '-', '*', '+' or '1.') and the text of the list item.

The number of spaces checked for depends on the document style in use, but the default is 1 space after any list marker:

* Foo
* Bar
* Baz

1. Foo
1. Bar
1. Baz

1. Foo
   * Bar
1. Baz

A document style may change the number of spaces after unordered list items and ordered list items independently, as well as based on whether the content of every item in the list consists of a single paragraph, or multiple paragraphs (including sub-lists and code blocks).

For example, the style guide at http://www.cirosantilli.com/markdown-styleguide/#spaces-after-marker specifies that 1 space after the list marker should be used if every item in the list fits within a single paragraph, but to use 2 or 3 spaces (for ordered and unordered lists respectively) if there are multiple paragraphs of content inside the list:

* Foo
* Bar
* Baz

vs.

*   Foo

    Second paragraph

*   Bar

or

1.  Foo

    Second paragraph

1.  Bar

To fix this, ensure the correct number of spaces are used after list marker for your selected document style.

MD031 - Fenced code blocks should be surrounded by blank lines

Tags: code, blank_lines

Aliases: blanks-around-fences

This rule is triggered when fenced code blocks are either not preceded or not followed by a blank line:

Some text
```
Code block
```

```
Another code block
```
Some more text

To fix this, ensure that all fenced code blocks have a blank line both before and after (except where the block is at the beginning or end of the document):

Some text

```
Code block
```

```
Another code block
```

Some more text

Rationale: Aside from aesthetic reasons, some parsers, including kramdown, will not parse fenced code blocks that don't have blank lines before and after them.

MD032 - Lists should be surrounded by blank lines

Tags: bullet, ul, ol, blank_lines

Aliases: blanks-around-lists

This rule is triggered when lists (of any kind) are either not preceded or not followed by a blank line:

Some text
* Some
* List

1. Some
2. List
Some text

To fix this, ensure that all lists have a blank line both before and after (except where the block is at the beginning or end of the document):

Some text

* Some
* List

1. Some
2. List

Some text

Rationale: Aside from aesthetic reasons, some parsers, including kramdown, will not parse lists that don't have blank lines before and after them.

Note: List items without hanging indents are a violation of this rule; list items with hanging indents are okay:

* This is
not okay

* This is
  okay

MD033 - Inline HTML

Tags: html

Aliases: no-inline-html

Parameters: allowed_elements (array of string; default empty)

This rule is triggered whenever raw HTML is used in a markdown document:

<h1>Inline HTML header</h1>

To fix this, use 'pure' markdown instead of including raw HTML:

# Markdown header

Rationale: Raw HTML is allowed in markdown, but this rule is included for those who want their documents to only include "pure" markdown, or for those who are rendering markdown documents in something other than HTML.

Note: To allow specific HTML elements, use the 'allowed_elements' parameter.

MD034 - Bare URL used

Tags: links, url

Aliases: no-bare-urls

This rule is triggered whenever a URL is given that isn't surrounded by angle brackets:

For more information, see http://www.example.com/.

To fix this, add angle brackets around the URL:

For more information, see <http://www.example.com/>.

Rationale: Without angle brackets, the URL isn't converted into a link in many markdown parsers.

Note: if you do want a bare URL without it being converted into a link, enclose it in a code block, otherwise in some markdown parsers it will be converted:

`http://www.example.com`

MD035 - Horizontal rule style

Tags: hr

Aliases: hr-style

Parameters: style ("consistent", "---", "***", or other string specifying the horizontal rule; default "consistent")

This rule is triggered when inconsistent styles of horizontal rules are used in the document:

---

- - -

***

* * *

****

To fix this, ensure any horizontal rules used in the document are consistent, or match the given style if the rule is so configured:

---

---

Note: by default, this rule is configured to just require that all horizontal rules in the document are the same, and will trigger if any of the horizontal rules are different than the first one encountered in the document. If you want to configure the rule to match a specific style, the parameter given to the 'style' option is a string containing the exact horizontal rule text that is allowed.

MD036 - Emphasis used instead of a header

Tags: headers, emphasis

Aliases: no-emphasis-as-header

Parameters: punctuation (string; default ".,;:!?")

This check looks for instances where emphasized (i.e. bold or italic) text is used to separate sections, where a header should be used instead:

**My document**

Lorem ipsum dolor sit amet...

_Another section_

Consectetur adipiscing elit, sed do eiusmod.

To fix this, use markdown headers instead of emphasized text to denote sections:

# My document

Lorem ipsum dolor sit amet...

## Another section

Consectetur adipiscing elit, sed do eiusmod.

Note: this rule looks for single line paragraphs that consist entirely of emphasized text. It won't fire on emphasis used within regular text, multi-line emphasized paragraphs, and paragraphs ending in punctuation. Similarly to rule MD026, you can configure what characters are recognized as punctuation.

MD037 - Spaces inside emphasis markers

Tags: whitespace, emphasis

Aliases: no-space-in-emphasis

This rule is triggered when emphasis markers (bold, italic) are used, but they have spaces between the markers and the text:

Here is some ** bold ** text.

Here is some * italic * text.

Here is some more __ bold __ text.

Here is some more _ italic _ text.

To fix this, remove the spaces around the emphasis markers:

Here is some **bold** text.

Here is some *italic* text.

Here is some more __bold__ text.

Here is some more _italic_ text.

Rationale: Emphasis is only parsed as such when the asterisks/underscores aren't completely surrounded by spaces. This rule attempts to detect where they were surrounded by spaces, but it appears that emphasized text was intended by the author.

MD038 - Spaces inside code span elements

Tags: whitespace, code

Aliases: no-space-in-code

This rule is triggered on code span elements that have spaces right inside the backticks:

` some text `

`some text `

` some text`

To fix this, remove the spaces inside the codespan markers:

`some text`

MD039 - Spaces inside link text

Tags: whitespace, links

Aliases: no-space-in-links

This rule is triggered on links that have spaces surrounding the link text:

[ a link ](http://www.example.com/)

To fix this, remove the spaces surrounding the link text:

[a link](http://www.example.com/)

MD040 - Fenced code blocks should have a language specified

Tags: code, language

Aliases: fenced-code-language

This rule is triggered when fenced code blocks are used, but a language isn't specified:

```
#!/bin/bash
echo Hello world
```

To fix this, add a language specifier to the code block:

```bash
#!/bin/bash
echo Hello world
```

MD041 - First line in file should be a top level header

Tags: headers

Aliases: first-line-h1

Parameters: level (number; default 1)

This rule is triggered when the first line in the file isn't a top level (h1) header:

```
This is a file without a header
```

To fix this, add a header to the top of your file:

```
# File with header

This is a file with a top level header
```

Note: The level parameter can be used to change the top level (ex: to h2) in cases where an h1 is added externally.

MD042 - No empty links

Tags: links

Aliases: no-empty-links

This rule is triggered when an empty link is encountered:

[an empty link]()

To fix the violation, provide a destination for the link:

[a valid link](https://example.com/)

Empty fragments will trigger this rule:

[an empty fragment](#)

But non-empty fragments will not:

[a valid fragment](#fragment)

MD043 - Required header structure

Tags: headers

Aliases: required-headers

Parameters: headers (array of string; default null for disabled)

This rule is triggered when the headers in a file do not match the array of headers passed to the rule. It can be used to enforce a standard header structure for a set of files.

To require exactly the following structure:

# Head
## Item
### Detail

Set the headers parameter to:

[
    "# Head",
    "## Item",
    "### Detail"
]

To allow optional headers as with the following structure:

# Head
## Item
### Detail (optional)
## Foot
### Notes (optional)

Use the special value "*" meaning "one or more unspecified headers" and set the headers parameter to:

[
    "# Head",
    "## Item",
    "*",
    "## Foot",
    "*"
]

When an error is detected, this rule outputs the line number of the first problematic header (otherwise, it outputs the last line number of the file).

Note that while the headers parameter uses the "## Text" ATX header style for simplicity, a file may use any supported header style.