Deleting certain integers from string list

Question

I have a list of strings:

lis = {"a","1","b","2","c","3","a","d","4"}

and would like to get:

res = {"a","b","2","c","3","a","d","4"}

where each occurrence of "a" that is immediately followed by (a string representation of) an integer, that integer is deleted from the list. ToExpression followed by IntegerQ seems inefficient, would be grateful for thoughts.

Answer 1

SequenceReplace:

SequenceReplace[lis, {"a", _?(StringMatchQ[NumberString])} :> "a"]

 {"a", "b", "2", "c", "3", "a", "d", "4"}

Also

SequenceReplace[lis, {"a", _?(IntegerQ @* ToExpression)} :> "a"]

 {"a", "b", "2", "c", "3", "a", "d", "4"}

Split + ReplaceAll

Flatten[Split[lis, # == "a" &] /. {"a", _?(IntegerQ@*ToExpression) } :> "a"]

 {"a", "b", "2", "c", "3", "a", "d", "4"}

Answer 2

The following works for your example. But I am not sure that it will work for you if your example is not descriptive enough for some more general situation you have in mind.

lis = {"a", "1", "b", "2", "c", "3", "a", "d", "4"};
Flatten[Partition[lis, UpTo[2]] /. {"a", "1"} -> "a"]

{"a", "b", "2", "c", "3", "a", "d", "4"}