You can require a space or end of string after your word, and you can make parts of your word(s) optional using the (?:...)? construct:

/^an?$/
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will match any string that starts with a and maybe has a b following.

/^a(?:n)?$/
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is the same, but using grouping parentheses.

/^a(?:n(?:g)?)?$/
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will match a, an or ang

As you can see, as the words get longer, the nesting gets deeper. Most likely, you will want to construct these word-matchers using some code instead of hand-crafting them.