pity that conditional formats do not allow for array formulas
(at least I think they don't)
but, you can try this:
<TABLE style="WIDTH: 48pt; BORDER-COLLAPSE: collapse" cellSpacing=0 cellPadding=0 width=64 border=0><COLGROUP><COL style="WIDTH: 48pt" width=64><TBODY><TR style="HEIGHT: 15pt" height=20><TD class=xl63 style="BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: windowtext 0.5pt solid; BACKGROUND: #c0504d; BORDER-LEFT: windowtext 0.5pt solid; WIDTH: 48pt; BORDER-BOTTOM: windowtext 0.5pt solid; HEIGHT: 15pt; mso-pattern: black none" width=64 height=20>a</TD></TR><TR style="HEIGHT: 15pt" height=20><TD class=xl63 style="BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: windowtext; BORDER-LEFT: windowtext 0.5pt solid; BORDER-BOTTOM: windowtext 0.5pt solid; HEIGHT: 15pt; BACKGROUND-COLOR: transparent" height=20>a1</TD></TR><TR style="HEIGHT: 15pt" height=20><TD class=xl63 style="BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: windowtext; BORDER-LEFT: windowtext 0.5pt solid; BORDER-BOTTOM: windowtext 0.5pt solid; HEIGHT: 15pt; BACKGROUND-COLOR: transparent" align=right height=20>1</TD></TR><TR style="HEIGHT: 15pt" height=20><TD class=xl63 style="BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: windowtext; BORDER-LEFT: windowtext 0.5pt solid; BORDER-BOTTOM: windowtext 0.5pt solid; HEIGHT: 15pt; BACKGROUND-COLOR: transparent" height=20>a2342</TD></TR></TBODY></TABLE>
after selecting Column A, enter this conditional format formula:
=AND(SUBSTITUTE(SUBSTITUTE(SUBSTITUTE(SUBSTITUTE(SUBSTITUTE(SUBSTITUTE(SUBSTITUTE(SUBSTITUTE(SUBSTITUTE(A1,1,""),2,""),3,""),4,""),5,""),6,""),7,""),8,""),9,"")=A1,A1<>"")
yuck, hope someone else has a better solution, but if not this should work for ya.