Regular expression without "Double or more" spaces

Question

I need a regular expression that allows:

1. You cannot put a space at the beginning and end of a line
2. There can only be one space between words

I had enough for something like "^[АЯ-Ёа-яё0-9' ']+$", - but that's not what I need.

Answer 1

This should work:

^(?! )(?!.*  )(?!.* $)[^\s].*$

Here's a breakdown of the expression:

• ^: Asserts the start of the line.
• (?! ): Negative lookahead to disallow a space at the beginning of the line.
• (?!.* ): Negative lookahead to disallow two or more consecutive spaces within the string.
• (?!.* $): Negative lookahead to disallow a space at the end of the line.
• [^\s]: Match any non-whitespace character.
• .*: Match any character (except a newline) 0 or more times.
• $: Asserts the end of the line.

I have put together a mini test on regex101.com.