Question

I want to convert the 5 space indentation in a python file to 4 space indentation. I want the command to do the following

remove a single space in all the lines which starts with a space followed by characters.

I issued the command %s/^\ [a-zA-Z]*// which seems to work. Later i figured out that the command should actually be

remove a single space in all the lines which starts with a space followed by any number of spaces followed by characters.

However still i am not able to figure out how the command(above) is working. It should basically report error for the following stating pattern not found but still it works.

class H:
     def __init__():
         hell()

Answer 1

It’s working because * means “match zero or more of the previous atom”. In your case, it’s matching zero. You probably wanted to use \+ instead which means “match one or more of the previous atom”.

In actuality, you could have just dropped the * entirely because just a space followed by a single character would have matched what you were originally searching for. There are better regular expressions for what you’re trying to accomplish, but that’s not what you’re asking here.

Edit (clarification):

Your regex as it stands (^\ [a-zA-Z]*) translates to:

• ^: From the start of the line
• \: Match a space
• [a-zA-Z]: Followed by a letter
• *: Zero or more times (of the previous atom – a letter)