YAPE::Regex::Explain is a handy tool to explain it for you.

use YAPE::Regex::Explain;
$search = "\x8C\x95";
$encoding = q{ # Shift-JIS encoding
[\x00-\x7F] # ASCII/JIS-Roman
| [\x81-\x9F\xE0-\xFC][\x40-\x7E\x80-\xFC] # JIS X 0208:1997
| [\xA0-\xDF] # Half-width katakana
};
$regex = qr/^ (?:$encoding)*? $search/osx;
print  YAPE::Regex::Explain->new($regex)->explain;
[download]

Outputs:

The regular expression:

(?sx-im:^ (?:  # Shift-JIS encoding
[\x00-\x7F]  # ASCII/JIS-Roman
| [\x81-\x9F\xE0-\xFC][\x40-\x7E\x80-\xFC]  # JIS X 0208:1997
| [\xA0-\xDF]  # Half-width katakana
)*? от)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?sx-im:                 group, but do not capture (with . matching
                         \n) (disregarding whitespace and comments)
                         (case-sensitive) (with ^ and $ matching
                         normally):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  (?:                      group, but do not capture (0 or more times
                           (matching the least amount possible)):
----------------------------------------------------------------------
    [\x00-\x7F]              any character of: '\x00' to '\x7F'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    [\x81-\x9F\xE0-          any character of: '\x81' to '\x9F',
    \xFC]                    '\xE0' to '\xFC'
----------------------------------------------------------------------
    [\x40-\x7E\x80-          any character of: '\x40' to '\x7E',
    \xFC]                    '\x80' to '\xFC'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    [\xA0-\xDF]              any character of: '\xA0' to '\xDF'
----------------------------------------------------------------------
  )*?                      end of grouping
----------------------------------------------------------------------
  от                       'от'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

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