How can I consolidate my three-part regex into a single pattern in C#?

Below is one way to simplify the three alternatives into one pattern. The goal is to require that the search string (aFrom) is “connected” to other non‐whitespace characters on at least one side. In other words, aFrom should be preceded by a non‑whitespace character or be followed by one.

A compact solution is to use alternation where one branch asserts a non‑whitespace before aFrom and the other branch asserts a non‑whitespace after aFrom. For example:

CSHARPCopied
return new Regex(
    @"(?:(?<=\S)" + aFrom + @"|" + aFrom + @"(?=\S))", 
    aCaseSensitivity);

Explanation

• (?<=\S)
  A positive lookbehind that asserts a non‑whitespace character immediately precedes aFrom.

• (?=\S)
  A positive lookahead that asserts a non‑whitespace character immediately follows aFrom.

• Grouping and Alternation
  The entire pattern uses a non‑capturing group (?:(…|…)) that allows either condition to match. This means that if aFrom is attached on the left (or on both sides), it matches the first alternative; if it is attached on the right, it matches the second alternative.

Behavior

• If aFrom is embedded in a larger word (both sides non‑whitespace), the first alternative (?<=\S)aFrom will match.
• If aFrom is preceded by another word but followed by a space, again the first alternative matches.
• If aFrom is followed by another word but preceded by a space (or start of string), the second alternative aFrom(?=\S) will match.
• If aFrom is isolated (surrounded by whitespace or string boundaries on both sides), neither alternative’s lookaround succeeds and the regex does not match.

This compact regex should serve the same purpose as the original three alternatives while being easier to read and maintain.