<p>A string will be made up of certain symbols (ax,bx,dx,c,acc for example) and numbers.</p>

<p>ex:
ax 5 5
dx 3 acc
c ax bx</p>

<p>I want to replace one or all of the symbols (randomly) with another symbol of the same set. ie, replace one of {ax,bx,dx,c,acc} with one of {ax,bx,dx,c,acc}.</p>

<p>replacement example:
acc 5 5
dx 3 acc
c ax bx
or
c 5 5
dx 3 acc
c ax ax</p>

<p>Is there a way to do this with regexes? In Java? If so, which methods should I use?</p>

To answer the first question: no.
Since you are doing a random replace, regex will not help you, nothing about regex is random. * Since your strings are in an array, you don't need to find them with any pattern matching, so again regex isn't necessary.
**Edit: the question has been edited so it no longer says the strings are in an array. In this case, assuming they are all in one big string, you might build a regex to find the parts you want to replace, as shown in other answers.*