[?] Why doesn't Cases[{"a", "b"}, "a"~~___] return {"a"} instead of {}?

Questions
1

Why doesn’t

Cases[{"a", "b"}, "a"~~___]

return {“a”} instead of {}?

I used three underscores. More detailed, why does 62 work, but 61 not

In Cases[{"abcde", "b"}, "a"~~__]

Out[61]= {}

In[62]:= StringMatchQ["abcde", "a" ~~ __]

Out[62]= True
2

Paul,

You need either

StringCases[{"a", "b"}, "a" ~~ ___]

which returns

{{"a"}, {}}

or

Select[{"ah", "b", "a"}, StringMatchQ["a" ~~ ___]]

which gives what I think you want:

{"ah", "a"}

I do not believe that Cases can take a string pattern. For that you would need to use StringCases.

Regards

3

As Neil also said, you are mistaking regular patterns with string patterns. Cases takes a pattern (which can involve strings!), StringMatchQ takes string patterns.

Look at the documentation:

guide/Patterns
tutorial/StringPatterns

In your case you can also use:

Select[{"a", "b"}, StringStartsQ["a"]]
4

Duplicate MMa.SE thread with good discussion on the subject: