twig string filters dont work.

This is because you are trying to run a string filter on a render array which is not possible. Agreed docs could be better but you need to |render the array first before you run other filters :)

It sounds then like item.content is not marked as safe, not sure what type of value it is. kint(item.content) or dump(item.content) might be worth a peek.

Okay I see what you mean now. We may have to consider whether some filters can create a new Markup object but there would have to be limits there I think.

To get what you want in your case I think you could do {{ item.content['#text']|striptags }}

Edit: To give more background I forgot this morning when replying that rendering returns a Markup object. Once you print and manipulate that string it's no longer safe, that's why it's getting escaped.

It would be nice to preserve safeness on some filters, but the manipulation *could* open a security hole. I tested with striptags and it was hard to crack, but maybe that's just time and patience. A bit hesitant to recommend |raw, but if I knew where the source was coming from, I'd be likely using that, but like the Issue summary says,

Do we really want ppl to do |raw on the strings, but yeah that will happen if thats the only way.

In my case filters are working very well without using filter block. For example, if we use capitalize filter it will return first value with an uppercase. Let me explain it as an example. My problem gets resolved with this, as we can try it for a better approach.

{% set var = 'Hello Testing' %}

{# Print in UPPERCASE #}
{{ var|upper }} 

OUTPUT--> {# 'HELLO TESTING' #}

Apart from upper there are few other filters are also available to modify the capitalization, We should try them for a better approach and modify capitalization style of our strings.

<ul>
  {# Set test strings #}
  {% set strLower = 'this is for test' %}
  {% set strUpper = 'THIS IS FOR TEST' %}
  {% set strMixed = 'this Is for TEST' %}

  {# Apply Twig filters #}
  <li>upper: {{ strLower|upper }}</li> {# 'THIS IS FOR TEST' #}
  <li>lower: {{ strUpper|lower }}</li> {# 'this is for test' #}

  {# Apply filters provided by Craft #}
  <li>ucwords: {{ strLower|ucwords }}</li> {# 'This Is For Test' #}
  <li>ucfirst: {{ strLower|ucfirst }}</li> {# 'This is for test' #}
  <li>lcfirst: {{ strUpper|lcfirst }}</li> {# 'tHIS IS FOR TEST' #}      

  {# Combine filters for predictable application #}
  <li>ucwords: {{ strMixed|lower|ucwords }}</li> {# 'This Is For Test' #}
  <li>ucfirst: {{ strMixed|lower|ucfirst }}</li> {# 'This is for test' #}
  <li>lcfirst: {{ strMixed|upper|lcfirst }}</li> {# 'tHIS IS FOR TEST' #}

  {# Examples for possible use case #}
  <li>PascalCase: {{ strMixed|lower|ucwords|replace(' ', '') }}</li> {# 'ThisIsForTest' #}
  <li>camelCase: {{ strMixed|lower|ucwords|lcfirst|replace(' ', '') }}</li> {# 'thisIsForTest' #}
</ul>

Hope this helps you.

I'm going to close this, but thanks for the extra help showing @dharmdeep.addweb.

Not sure if this will help anyone, but the following fixed my issue.

Changed in the theme page.html.twig

{% if page.sidebar_first|trim %}

to

{% if page.sidebar_first|render|trim %}

THX, made my day!

However, i cannot figure out why e.g. the piped trim filter does not work without the additional piped render filter.

This worked for me thanks:

<h2 id="{{ item.content|render|replace({' ': '-'}) }}">Hello</h2>