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  1. #!/bin/bash
  2. s='```html   class1 class2
  3. html   class3 class4'
  4. sed -E '/^```/ s/[[:space:]]+/,/g' <<< "$s"
Success #stdin #stdout 0.01s 5532KB
comments (?)
stdin 
Standard input is empty
stdout 
```html,class1,class2
html   class3 class4
https://ideone.com/ROtYHw
language:
Bash (bash 5.0.3)
created:
5 years ago
visibility:
secret

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