How to show a specific lines from a post as a post summary in blogger?

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Javascript

HTML

I'm trying to create a blogger template for my own educational purpose. I need to show a specific lines from a post as a post summary in blogger. I tried two methods one of them is by the code given below, but problem is those scripts just shows the first n characters as a post summary but I need a particular content present in-between the paragraph as a post summary.

<b:if cond='data:blog.pageType != "item"'>     
    <img expr:src='data:post.thumbnailUrl'/><br/>
        <data:post.snippet/>
      </b:if>

The next method, I use individual search description for my post. so I decided to print the search description as my post summary and use this code

<b:if cond='data:blog.pageType != "item"'>     
        <img expr:src='data:post.thumbnailUrl'/><br/>
            <data:blog.metaDescription/>
          </b:if>

but again those code only print the description of the blog not the post. So is there any other way to show the specific line as a post summary.

What I have tried:

Method:1

<b:if cond='data:blog.pageType != "item"'>     
    <img expr:src='data:post.thumbnailUrl'/><br/>
        <data:post.snippet/>
      </b:if>

Method:2

<b:if cond='data:blog.pageType != "item"'>     
        <img expr:src='data:post.thumbnailUrl'/><br/>
            <data:blog.metaDescription/>
          </b:if>