Assert failure with ICU support

I happened to run into an assert failure by chance with ICU support.
Here is the query:

SELECT '1' SIMILAR TO '\൧';

The failure happens in lexescape(),

default:
assert(iscalpha(c));
FAILW(REG_EESCAPE); /* unknown alphabetic escape */
break;

Without ICU support, the same query just gives an error.

# SELECT '1' SIMILAR TO '\൧';
ERROR: invalid regular expression: invalid escape \ sequence

FWIW, I googled a bit and '൧' seems to be number 1 in Malayalam.

Thanks
Richard