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Why when trying to find whether one (out of several) patterns matches an expression - using StringMatchQ - all of them seem to be ignored and no matches are found? In this simple example I try to find out whether or not the 4-letter-long string "abcd" starts with an "a"

StringMatchQ[
  StartOfString ~~ "a" ~~ _ ~~ _ ~~ _]["abcd"]

True

However, when I try to test whether it starts with either an "a" or, say, a "b",

StringMatchQ[
      StartOfString ~~ "a" ~~ _ ~~ _ ~~ _|StartOfString ~~ "b" ~~ _ ~~ _ ~~ _]["abcd"]

False

Shouldn't it be true since the first of the two patterns is a match? Isn't this used to specify that either one pattern or the other is the pattern to be matched?

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    $\begingroup$ For that specific example StringMatchQ["a*" | "b*"]["abcd"] $\endgroup$
    – Rohit Namjoshi
    Commented Jan 27, 2020 at 22:41

1 Answer 1

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Wrap the patterns with parantheses:

StringMatchQ[(StartOfString ~~ "a" ~~ _ ~~ _ ~~ _) | 
  (StartOfString ~~ "b" ~~ _ ~~ _ ~~ _)]["abcd"]

True

Alternatively, use the input form of Alternatives:

StringMatchQ[Alternatives[StartOfString ~~ "a" ~~ _ ~~ _ ~~ _, 
   StartOfString ~~ "b" ~~ _ ~~ _ ~~ _]]["abcd"]

True

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