Join non-digit character strings

Question

I have a list:

lis = {"20","ab","Cd","10","E"}

in which I would like to join adjacent elements if they are letters and not digits, to give:

res = {"20","abCd","10","E"}

Again thanks for any help!

Answer 1

Maybe something like this:

StringJoin /@ SplitBy[lis, LetterQ]

Edit

That doesn't work if there are adjacent non-letter strings. Try this instead:

If[AllTrue[#, LetterQ], StringJoin[#], Splice[#]] & /@ SplitBy[lis, LetterQ]

Another option:

Flatten[SequenceSplit[lis, {str__?LetterQ} :> StringJoin[str]]]

Answer 2

lis = {"20", "ab", "Cd", "10", "E"}

Using SequenceReplace:

SequenceReplace[lis, {s__String?(StringFreeQ[DigitCharacter])} .. :> 
  StringJoin@s]

{"20", "abCd", "10", "E"}