Regex - How to prevent any string that starts with "de" but cannot use lookahead or lookbehind?

There's a solution without a lookahead or lookbehind, but you need to be able to use groups.

The idea there is to create a sort of "honeypot" that will match your negative results and keep only the results that do interest you.

In your case, that would write:

[dD][eE].*|(<your-regex>)

If the proposition is de<anything> (case insensitive here), it will match, but group(1) will be null.

On the other hand, matching diZ for instance would match not match what is before the or and would therefore fall into the group(1).

Finally, if the proposition doesn't start with de and doesn't match your regex, well, there will be no groups to get at all.

If you need to be sure that your proposition will match the whole provided string, you can update the regex thus:

^(?:[dD][eE].*|(<your-regex>))$

Note that ?: is not a lookahead of any kind, it serves to mark the group as non-capturing, so that <your-regex> will still be captured by group(1) (would become group(2) otherwise and the capture of a group is not always a transparent operation, performance-wise).

Simply ignore those characters:

[a-ce-z][a-df-z][a-gi-kwxyzWZXZ]

Make sure the flag is set to case insensitive. Also, [a-gi-kwxyzWZXZ] can then be modified to [a-gi-kwxyz].

EDIT:

As pointed out in this comment, the regex here won't support other words that start with d but are not followed by e. In this case, negative lookahead is a possible solution:

^(?!de)[a-z]+

This matches anything not starting with "DE" (case insensitive, without look arounds, allowing leading whitespace):

^ *+(?:[^Dd].|.[^Ee])<your regex for rest of input>

See live demo.

The possessive quantifier *+ used for whitespace prevents [^Dd] from being allowed to match a space via backtracking, making this regex hardened against leading spaces.

You can use an alternation excluding matching the d and D from the first character, or exclude matching the e as the second character.

Note that the pattern [a-zA-Z][a-z] matches at least 2 characters, so will the following pattern:

^(?:[abce-zABCE-Z][a-z]|[a-zA-Z][a-df-z]).*

• ^ Start of string
• (?: Non capture group
  • [abce-zABCE-Z][a-z] Match a char a-zA-Z without d and D followed by a lowercase char a-z
  • | or
  • [a-zA-Z][a-df-z] Match a char a-zA-Z followed by a lowercase chars a-z without e
• ) Close non capture grou
• .* Match 0+ times any char except a newline

Regex demo

Another option is to use word boundaries \b instead of an anchor ^

\b(?:[abce-zABCE-Z][a-z]|[a-zA-Z][a-df-z])[a-zA-Z]*\b

Regex demo