# Filters

Each job can have two filter stages configured, with one or more filters processed after each other:

- Applied to the downloaded page before diffing the changes (`filter`)

- Applied to the diff result before reporting the changes (`diff_filter`)

While creating your filter pipeline, you might want to preview what the filtered output looks like. You can do so by first configuring your job and then running urlwatch with the `--test-filter` command, passing in the index (from `--list`) or the URL/location of the job to be tested:

    urlwatch --test-filter 1   # Test the first job in the list
    urlwatch --test-filter https://example.net/  # Test the job with the given URL

The output of this command will be the filtered plaintext of the job, this is the output that will (in a real urlwatch run) be the input to the diff algorithm.

The `filter` is only applied to new content, the old content was already filtered when it was retrieved. This means that changes to `filter` are not visible when reporting unchanged contents (see Display for details), and the diff output will be between (old content with filter at the time old content was retrieved) and (new content with current filter).

Once urlwatch has collected at least 2 historic snapshots of a job (two different states of a webpage) you can use the command-line option `--test-diff-filter` to test your `diff_filter` settings; this will use historic data cached locally.

## Built-in filters

The list of built-in filters can be retrieved using:

    urlwatch --features

At the moment, the following filters are built-in:

- beautify: Beautify HTML

- css: Filter XML/HTML using CSS selectors

- csv2text: Convert CSV to plaintext

- element-by-class: Get all HTML elements by class

- element-by-id: Get an HTML element by its ID

- element-by-style: Get all HTML elements by style

- element-by-tag: Get an HTML element by its tag

- format-json: Convert to formatted json

- grep: Filter only lines matching a regular expression

- grepi: Remove lines matching a regular expression

- hexdump: Convert binary data to hex dump format

- html2text: Convert HTML to plaintext

- pdf2text: Convert PDF to plaintext

- pretty-xml: Pretty-print XML

- ical2text: Convert iCalendar to plaintext

- ocr: Convert text in images to plaintext using Tesseract OCR

- re.sub: Replace text with regular expressions using Python’s re.sub

- re.findall: Find all non-overlapping matches using Python’s re.findall

- reverse: Reverse input items

- sha1sum: Calculate the SHA-1 checksum of the content

- shellpipe: Filter using a shell command

- sort: Sort input items

- remove-duplicate-lines: Remove duplicate lines (case sensitive)

- strip: Strip leading and trailing whitespace

- striplines: Strip leading and trailing whitespace in each line

- xpath: Filter XML/HTML using XPath expressions

- jq: Filter, transform and extract values from JSON

## Picking out elements from a webpage

You can pick only a given HTML element with the built-in filter, for example to extract `<div id="something">.../<div>` from a page, you can use the following in your `urls.yaml`:

    url: http://example.org/idtest.html
    filter:
      - element-by-id: something

Also, you can chain filters, so you can run html2text on the result:

    url: http://example.net/id2text.html
    filter:
      - element-by-id: something
      - html2text

## Chaining multiple filters

The example urls.yaml file also demonstrates the use of built-in filters, here 3 filters are used: html2text, line-grep and whitespace removal to get just a certain info field from a webpage:

    url: https://example.net/version.html
    filter:
      - html2text
      - grep: "Current.*version"
      - strip

## Extracting only the `<body>` tag of a page

If you want to extract only the body tag you can use this filter:

    url: https://example.org/bodytag.html
    filter:
      - element-by-tag: body

## Filtering based on an XPath expression

To filter based on an XPath expression, you can use the `xpath` filter like so:

    url: https://example.net/xpath.html
    filter:
      - xpath: /html/body/marquee

This filters only the `<marquee>` elements directly below the `<body>` element, which in turn must be below the `<html>` element of the document, stripping out everything else.

See Microsoft’s XPath Examples page for some other examples. You can also find an XPath of an `<html>` node in the Chromium/Google Chrome developer tools by right clicking on the node and selecting `copy XPath`.

## Filtering based on CSS selectors

To filter based on a CSS selector, you can use the `css` filter like so:

    url: https://example.net/css.html
    filter:
      - css: ul#groceries > li.unchecked

This would filter only `<li class="unchecked">` tags directly below `<ul id="groceries">` elements.

Some limitations and extensions exist as explained in cssselect’s documentation.

## Using XPath and CSS filters with XML and exclusions

By default, XPath and CSS filters are set up for HTML documents. However, it is possible to use them for XML documents as well (these examples parse an RSS feed and filter only the titles and publication dates):

    url: https://example.com/blog/xpath-index.rss
    filter:
      - xpath:
          path: '//item/title/text()|//item/pubDate/text()'
          method: xml

    url: http://example.com/blog/css-index.rss
    filter:
      - css:
          selector: 'item > title, item > pubDate'
          method: xml
      - html2text: re

To match an element in an XML namespace, use a namespace prefix before the tag name. Use a `:` to separate the namespace prefix and the tag name in an XPath expression, and use a `|` in a CSS selector.

    url: https://example.net/feed/xpath-namespace.xml
    filter:
      - xpath:
          path: '//item/media:keywords/text()'
          method: xml
          namespaces:
            media: http://search.yahoo.com/mrss/

    url: http://example.org/feed/css-namespace.xml
    filter:
      - css:
          selector: 'item > media|keywords'
          method: xml
          namespaces:
            media: http://search.yahoo.com/mrss/
      - html2text

Alternatively, use the XPath expression `//*[name()='<tag_name>']` to bypass the namespace entirely.

Another useful option with XPath and CSS filters is `exclude`. Elements selected by this `exclude` expression are removed from the final result. For example, the following job will not have any `<a>` tag in its results:

    url: https://example.org/css-exclude.html
    filter:
      - css:
          selector: body
          exclude: a

## Limiting the returned items from a CSS Selector or XPath

If you only want to return a subset of the items returned by a CSS selector or XPath filter, you can use two additional subfilters:

- `skip`: How many elements to skip from the beginning (default: 0)

- `maxitems`: How many elements to return at most (default: no limit)

For example, if the page has multiple elements, but you only want to select the second and third matching element (skip the first, and return at most two elements), you can use this filter:

    url: https://example.net/css-skip-maxitems.html
    filter:
      - css:
          selector: div.cpu
          skip: 1
          maxitems: 2

### Dealing with duplicated results

If you get multiple results on one page, but you only expected one (e.g. because the page contains both a mobile and desktop version in the same HTML document, and shows/hides one via CSS depending on the viewport size), you can use `maxitems: 1` to only return the first item.

## Fixing list reorderings with CSS Selector or XPath filters

In some cases, the ordering of items on a webpage might change regularly without the actual content changing. By default, this would show up in the diff output as an element being removed from one part of the page and inserted in another part of the page.

In cases where the order of items doesn’t matter, it’s possible to sort matched items lexicographically to avoid spurious reports when only the ordering of items changes on the page.

The subfilter for `css` and `xpath` filters is `sort`, and can be `true` or `false` (the default):

    url: https://example.org/items-random-order.html
    filter:
      - css:
          selector: span.item
          sort: true

## Filtering PDF documents

To monitor the text of a PDF file, you use the pdf2text filter. It requires the installation of the pdftotext library and any of its OS-specific dependencies.

This filter must be the first filter in a chain of filters, since it consumes binary data and outputs text data.

    url: https://example.net/pdf-test.pdf
    filter:
      - pdf2text
      - strip

If the PDF file is password protected, you can specify its password:

    url: https://example.net/pdf-test-password.pdf
    filter:
      - pdf2text:
          password: urlwatchsecret
      - strip

## Dealing with CSV input

The `csv2text` filter can be used to turn CSV data to a prettier textual representation This is done by supplying a `format_string` which is a python format string.

If the CSV has a header, the format string should use the header names lowercased. For example, let’s say we have a CSV file containing data like this:

    Name;Company
    Smith;Initech
    Doe;Initech

A possible format string for the above CSV (note the lowercase keys):

    Mr {name} works at {company}

If there is no header row, you will need to use the numeric array notation:

    Mr {0} works at {1}

You can force the use of numeric indices with the flag `ignore_header`.

The key `has_header` can be used to force use the first line or first ignore the first line as header, otherwise csv.Sniffer will be used.

## Sorting of webpage content

Sometimes a web page can have the same data between comparisons but it appears in random order. If that happens, you can choose to sort before the comparison.

    url: https://example.net/sorting.txt
    filter:
      - sort

The sort filter takes an optional `separator` parameter that defines the item separator (by default sorting is line-based), for example to sort text paragraphs (text separated by an empty line):

    url: http://example.org/paragraphs.txt
    filter:
      - sort:
          separator: "\n\n"

This can be combined with a boolean `reverse` option, which is useful for sorting and reversing with the same separator (using `%` as separator, this would turn `3%2%4%1` into `4%3%2%1`):

    url: http://example.org/sort-reverse-percent.txt
    filter:
      - sort:
          separator: '%'
          reverse: true

## Reversing of lines or separated items

To reverse the order of items without sorting, the `reverse` filter can be used. By default it reverses lines:

    url: http://example.com/reverse-lines.txt
    filter:
      - reverse

This behavior can be changed by using an optional separator string argument (e.g. items separated by a pipe (`|`) symbol, as in `1|4|2|3`, which would be reversed to `3|2|4|1`):

    url: http://example.net/reverse-separator.txt
    filter:
      - reverse: '|'

Alternatively, the filter can be specified more verbose with a dict. In this example `"\n\n"` is used to separate paragraphs (items that are separated by an empty line):

    url: http://example.org/reverse-paragraphs.txt
    filter:
      - reverse:
          separator: "\n\n"

## Watching Github releases and Gitlab tags

This is an example how to watch the GitHub “releases” page for a given project for the latest release version, to be notified of new releases:

    url: https://github.com/tulir/gomuks/releases
    filter:
      - xpath:
          path: //*[@class="Link--primary Link"]
          maxitems: 1
      - html2text:

This is the corresponding version for Github tags:

    url: https://github.com/thp/urlwatch/tags
    filter:
      - xpath:
          path: //*[@class="Link--primary Link"]
          maxitems: 1
      - html2text:

and for Gitlab tags:

    url: https://gitlab.com/chinstrap/gammastep/-/tags
    filter:
      - xpath: (//a[contains(@class,"item-title ref-name")])[1]
      - html2text

Alternatively, `jq` can be used for filtering:

    url: https://api.github.com/repos/voxpupuli/puppet-rundeck/tags
    filter:
      - jq: '.[0].name'

## Find, remove or replace text using regular expressions

You can use `re.sub` and `re.findall` to apply regular expressions.

`re.sub` can be used to remove or replace all non-overlapping instances of matched text. The following example applies the filter 3 times:

1. Just specifying a string as the value will replace the matches with the empty string.

2. Simple patterns can be replaced with another string using “pattern” as the expression and “repl” as the replacement.

3. You can use groups (`()`) and back-reference them with `\1` (etc..) to put groups into the replacement string.

`repl` defaults to the empty string, which will remove matched strings.

    url: https://example.com/regex-substitute.html
    filter:
        - re.sub: '\s*href="[^"]*"'
        - re.sub:
            pattern: '<h1>'
            repl: 'HEADING 1: '
        - re.sub:
            pattern: '</([^>]*)>'
            repl: '<END OF TAG \1>'

`re.findall` can be used to find all non-overlapping matches of a regular expression. Each match is output on its own line. The following example applies the filter twice:

1. It uses a group (`()`) and back-reference (`\1`) to extract a date from the input string.

2. It breaks the numbers in the date out into separate lines.

If `repl` is not specified, the full match will be included in the output.

    url: https://example.com/regex-findall.html
    filter:
        - re.findall:
            pattern: 'The next draw is on (\d{4}-\d{2}-\d{2}).'
            repl: '\1'
        - re.findall: '\d+'

Note: When using HTML or XML, it is usually better to use CSS selectors or XPATH expressions. HTML and XML cannot be parsed properly using regular expressions. If the CSS selector or XPATH cannot provide the targeted selection required, using an `html2text` filter first then using `re.findall` can be a good pattern.

If you want to enable flags (e.g. `re.MULTILINE`) in `re.sub` or `re.findall` filters, use an “inline flag”, here are some examples:

- `re.MULTILINE`: `(?m)` (Makes `^` match start-of-line and `$` match end-of-line)

- `re.DOTALL`: `(?s)` (Makes `.` also match a newline)

- `re.IGNORECASE`: `(?i)` (Perform case-insensitive matching)

This allows you, for example, to remove all leading spaces (only space character and tab):

    url: http://example.com/leading-spaces.txt
    filter:
      - re.sub: '(?m)^[ \t]*'

## Using a shell script as a filter

While the built-in filters are powerful for processing markup such as HTML and XML, in some cases you might already know how you would filter your content using a shell command or shell script. The `shellpipe` filter allows you to start a shell and run custom commands to filter the content.

The text data to be filtered will be written to the standard input (`stdin`) of the shell process and the filter output will be taken from the shell’s standard output (`stdout`).

For example, if you want to use `grep` tool with the case insensitive matching option (`-i`) and printing only the matching part of the line (`-o`), you can specify this as `shellpipe` filter:

    url: https://example.net/shellpipe-grep.txt
    filter:
      - shellpipe: "grep -i -o 'price: <span>.*</span>'"

This feature also allows you to use sed(1), awk(1) and perl(1) one-liners for text processing (of course, any text tool that works in a shell can be used). For example, this awk(1) one-liner prepends the line number to each line:

    url: https://example.net/shellpipe-awk-oneliner.txt
    filter:
      - shellpipe: awk '{ print FNR " " $0 }'

You can also use a multi-line command for a more sophisticated shell script (`|` in YAML denotes the start of a text block):

    url: https://example.org/shellpipe-multiline.txt
    filter:
      - shellpipe: |
          FILENAME=`mktemp`
          # Copy the input to a temporary file, then pipe through awk
          tee $FILENAME | awk '/The numbers for (.*) are:/,/The next draw is on (.*)./'
          # Analyze the input file in some other way
          echo "Input lines: $(wc -l $FILENAME | awk '{ print $1 }')"
          rm -f $FILENAME

Within the `shellpipe` script, two environment variables will be set for further customization (this can be useful if you have an external shell script file that is used as filter for multiple jobs, but needs to treat each job in a slightly different way):

Environment variable | Contents
--- | ---
`$URLWATCH_JOB_NAME` | The name of the job (`name` key in jobs YAML)
`$URLWATCH_JOB_LOCATION` | The URL of the job, or command line (for shell jobs)

## Converting text in images to plaintext

The `ocr` filter uses the Tesseract OCR engine to convert text in images to plain text. It requires two Python modules to be installed: pytesseract and Pillow. Any file formats supported by Pillow (PIL) are supported.

This filter must be the first filter in a chain of filters, since it consumes binary data and outputs text data.

    url: https://example.net/ocr-test.png
    filter:
      - ocr:
          timeout: 5
          language: eng
      - strip

The subfilters `timeout` and `language` are optional:

- `timeout`: Timeout for the recognition, in seconds (default: 10 seconds)

- `language`: Text language (e.g. `fra` or `eng+fra`, default: `eng`)

## Filtering JSON response data using `jq` selectors

The `jq` filter uses the Python bindings for jq, a lightweight JSON processor. Use of this filter requires the optional jq Python module to be installed.

    url: https://example.net/jobs.json
    filter:
       - jq:
          query: '.[].title'

The subfilter `query` is optional:

- `query`: A valid `jq` filter string.

Supports aggregations, selections, and the built-in operators like `length`. For more information on the operations permitted, see the jq Manual.