Filters

Filters are currently used in two stages of processing:

  • 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
  • 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
  • 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
  • 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 (see Microsoft’s XPath Examples page for some other examples):

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.

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.

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

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](https://docs.python.org/3/library/string.html#format-string-syntax). If the CSV has a header, the format string should use the header names (lowercased). Example:

Name | Company |

|--|–| | Smith | Initech | | Doe | Initech |

Format string for the above CSV: Mr {name} works at {company} (Note the lowercase). If there is no header row, you will need to use the numeric array notation: Mr {0} works at {1}. You can also use numeric array on CSV with headers with the flag ignore_header. has_header can be used to force use the first line or first ignore the first line as header, otherwise [csv.Sniffer](https://docs.python.org/3/library/csv.html#csv.Sniffer) would 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/thp/urlwatch/releases
filter:
  - xpath: '(//div[contains(@class,"release-timeline-tags")]//h4)[1]/a'
  - html2text: re
  - strip

This is the corresponding version for Gitlab tags:

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

Remove or replace text using regular expressions

Just like Python’s re.sub function, there’s the possibility to apply a regular expression and either remove of replace the 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.

All features are described in Python’s re.sub documentation (the pattern and repl values are passed to this function as-is, with the value of repl defaulting to the empty string).

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>'

If you want to enable certain flags (e.g. re.MULTILINE) in the call, this is possible by inserting an “inline flag” documented in flags in re.compile, 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, awk and perl one-liners for text processing (of course, any text tool that works in a shell can be used). For example, this awk 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.