Look into \1 and related, the backreference equivalents for $1 etc. /(\w)\1/ will match repeated letters (and digits and underscore), and /(\w)\1/i will do that case insensitively, thus the former will math "ww" or "WW" but not "wW", while the latter will match "wW" too.

For more on this, check out the classic article by Tom Christiansen, FMTEYEWTK about regular expressions.

p.s. Make sure that you backreference number agrees with you used capturing parens. If you want to wrap parens around the whole match, you'll have to replace the \1 with \2:

  /((\w)\2)/
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BTW To know the correct number to use, just count the opening parens. So the outer parens are number 1, the inner parens are number 2.