Looking for valid URLs

Valid URLs are protocol://someadress, for this example we are going to use a file with this content:

• http://www.google.com
• http:/www.google.com
• ftp://123.44.5.22
• https://255.122.22.11
• htt://google.com
• http://google.com
• file://somefile
• file//another-file

We will support these protocols:

• http
• https
• ftp

So, the expression for find these values is quite simple:

$ grep -P "^(https?|ftp|file)://.+$" file.txt

• http://www.google.com
• ftp://123.44.5.22
• https://255.122.22.11
• http://google.com
• file://somefile

Where:

- ^               : at the beginning
- https?|ftp|file : http or https if no, find ftp if no find file
- ://             : ://  ( self explained )
- .+              : any characters
- $               : at the end

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