These are the characters that are matched by \w in PCRE v7.8
(regardless of whether you use the u modifier):

Array
        (
            [0] => 0
            [1] => 1
            [2] => 2
            [3] => 3
            [4] => 4
            [5] => 5
            [6] => 6
            [7] => 7
            [8] => 8
            [9] => 9
            [10] => A
            [11] => B
            [12] => C
            [13] => D
            [14] => E
            [15] => F
            [16] => G
            [17] => H
            [18] => I
            [19] => J
            [20] => K
            [21] => L
            [22] => M
            [23] => N
            [24] => O
            [25] => P
            [26] => Q
            [27] => R
            [28] => S
            [29] => T
            [30] => U
            [31] => V
            [32] => W
            [33] => X
            [34] => Y
            [35] => Z
            [36] => _
            [37] => a
            [38] => b
            [39] => c
            [40] => d
            [41] => e
            [42] => f
            [43] => g
            [44] => h
            [45] => i
            [46] => j
            [47] => k
            [48] => l
            [49] => m
            [50] => n
            [51] => o
            [52] => p
            [53] => q
            [54] => r
            [55] => s
            [56] => t
            [57] => u
            [58] => v
            [59] => w
            [60] => x
            [61] => y
            [62] => z
            [63] => ª
            [64] => µ
            [65] => º
            [66] => À
            [67] => Á
            [68] => Â
            [69] => Ã
            [70] => Ä
            [71] => Å
            [72] => Æ
            [73] => Ç
            [74] => È
            [75] => É
            [76] => Ê
            [77] => Ë
            [78] => Ì
            [79] => Í
            [80] => Î
            [81] => Ï
            [82] => Ð
            [83] => Ñ
            [84] => Ò
            [85] => Ó
            [86] => Ô
            [87] => Õ
            [88] => Ö
            [89] => Ø
            [90] => Ù
            [91] => Ú
            [92] => Û
            [93] => Ü
            [94] => Ý
            [95] => Þ
            [96] => ß
            [97] => à
            [98] => á
            [99] => â
            [100] => ã
            [101] => ä
            [102] => å
            [103] => æ
            [104] => ç
            [105] => è
            [106] => é
            [107] => ê
            [108] => ë
            [109] => ì
            [110] => í
            [111] => î
            [112] => ï
            [113] => ð
            [114] => ñ
            [115] => ò
            [116] => ó
            [117] => ô
            [118] => õ
            [119] => ö
            [120] => ø
            [121] => ù
            [122] => ú
            [123] => û
            [124] => ü
            [125] => ý
            [126] => þ
            [127] => ÿ
        )