Regular Expression: Match a string not immediately preceded by or followed by a alphanumeric character

Question

When searching for d

For example

• " D" = true
• " D " = true
• "D" = true
• "[D!" = true
• "ad!" = false
• "sadness" = false
• "sa d!ness" = true

Answer 1

You're probably looking for word boundary anchors:

\bd\b

matches d only if it isn't adjacent to other alphanumerics.

Note that the definition of "alphanumeric" varies between regex engines. Most define them as the character set [A-Za-z0-9_], but some also include non-ASCII letters/digits.

Answer 2

You can use lookarounds:

(?<![A-Za-z0-9])[Dd](?![A-Za-z0-9])

Which means match d or D which is not preceded or followed by [A-Za-z0-9].

RegEx Demo