Eliminate "two or more" spaces in regular expression

Question

I need a regular expression to allow:

1. Cannot place spaces at the beginning and end of the line
2. There can only be one space between words

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

Answer 1

This should work:

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

The following is a breakdown of the expression:

• ^: Assert the beginning of the line.
• (?! ): Negates lookahead and prohibits spaces at the beginning of the line.
• (?!.*): Negative lookahead, two or more consecutive spaces are not allowed in the string.
• (?!.* $): Negates lookahead and does not allow spaces at the end of the line.
• [^\s]: Matches any non-whitespace characters.
• .*: Matches any character (except newline) 0 or more times.
• $: Assert the end of line.

I ran a small test on regex101.com.