Regex: word boundary but for white space, beginning of line or end of line only

Question

I am looking for some word boundary to cover those 3 cases:

1. beginning of string
2. end of string
3. white space

Is there something like that since \b covers also -,/ etc.?

Would like to replace \b in this pattern by something described above:

(\b\d*\sx\s|\b\d*x|\b)

Answer 1

OK, so your real question is:

How do I match a unit, optionally preceded by a quantity, but only if there is either nothing or a space right before the match?

Use

 (?<!\S)\b(?:\d+\s*x\s*)?\d+(?:\.\d+)?\s*ml\b

Explanation

(?<!\S): Assert that it's impossible to match a non-space character before the match.

\b: Match a word boundary

(?:\d+\s*x\s*)?: Optionally match a quantifier (integers only)

\d+(?:\.\d+)?: Match a number (decimals optional)

\s*ml\b: Match ml, optionally preceded by whitespace.