DuckDuckGo + Wolfram|Alpha
This page contains open-sourced code that we use to integrate WolframAlpha (a knowledge engine) into DuckDuckGo (a search engine). It is also on github.
This integration consists of the following Perl regular expressions to identify query types that WolframAlpha answers well. These queries then get proxied via the WWW::WolframAlpha Perl module, which we also maintain on CPAN.
Some variables used
$into_qr = qr/\s(?:\-|en|in|to|in ?to)\s/i;

$q_check = $q;
$q_check =~ s/^\!//;
$q_check =~ s/\?$//;

$q_escape = encode_for_link($q);

$q_internal is $q minus special chars
Chemical Formulas
Queries consisting fully of one or more chemical element blocks, e.g. Si4 or O8.
$q =~ /^!?(?:(?:\()*(?=[ABCDEFGHIKLMNOPRSTUVWXYZ])(?:S[gnbmreic]?|N[eoabdip]?|G[dae]|B[ariehk]?|U(?:u[ptsohq])?|P[troamdbu]?|M[dgton]|A[ltrmgcus]|I[nr]?|R[bengaufh]|C[rnldumofsae]?|L[ruia]|E[rus]|T[hlacemib]|D[ys]|H[eogsaf]?|F[emr]?|Kr?|Yb?|Z[rn]|Os?|[VW]|Xe)(?:\d|\))*)+$/ && $q =~ /(\d+)/ && $1<20
Examples
Calendar
Calendars.
$q_check =~ /\bcal[ea]nd[ae]r\b/i && $q_check =~ /\b(?:\d{4}|jan(?:uary|)|feb(?:r?uary|)|mar(?:ch|)|apr(?:il|)|may|june?|july?|aug(?:ust|)|sep(?:tember|)|oct(?:ober|)|nov(?:ember|)|dec(?:ember|))\b/
Conversions
Queries with a number, unit, conversion word and conversion target.
$q_check =~ /^(?:convert\s|)[^a-z]?\d[\d\,\.]*\s?.{1,10}$into_qr(?:.{1,15}|[^\s]+)$/i

$q_check =~ /^.{1,10}\s*[^a-z]?\d[\d\,\.]*$into_qr(?:.{1,15}|[^\s]+)$/i

$q_escape =~ s/(\b|\%20)cad(\b|\%20)/$1canadian dollars$2/i;
$q_escape =~ s/(\%20)en(\%20)/$1in$2/i;
$q_escape =~ s/(\b|\%20)jst(\b|\%20)/$1jt$2/i;
$q_escape =~ s/(\b|\%20)bht(\b|\%20)/$1thb$2/i;
$q_escape =~ s/(\d)\%2C(\d{2})(?!\d)/$1.$2/i;
Currency
Currency conversions not caught by above.
my $currency_qr = qr/\b(?=[abcdefghijklmnopqrstuvwxyz])(?:a(?:o[an]|r[asp]|fa|ll|[mue]d|[wn]g|zm)|d(?:zd|kk|jf|op)|u(?:a[hk]|sd|[gz]s|yu|kp)|e(?:ur|gp|ek|tb|cs|rn)|x(?:cd|[apo]f|eu)|n(?:ok|[za]d|pr|ic|gn)|b(?:o[bp]|[shbzmn]d|dt|yr|am|wp|[rg]l|if|tn|uk)|k(?:hr|[yw]d|mf|zt|[eg]s|[pr]w)|c(?:l[fp]|[as]d|ve|ny|[ou]p|rc|zk|hf|dz)|t(?:pe|[wtn]d|jr|zs|hb|op|ry|mm)|f(?:kp|jd)|g(?:[my]d|el|hc|[ibw]p|tq|ns|qe)|h(?:r[kd]|tg|nl|kd|uf)|i(?:sk|[ndr]r|qd|ls)|j(?:[mo]d|py)|l(?:s[lm]|ak|[vt]l|bp|[ry]d|kr)|m(?:x[np]|op|[ka]d|[gl]f|[wm]k|[yvu]r|ro|dl|nt|zm)|p(?:e[ins]|l[nz]|kr|gk|yg|hp|ab)|r(?:u[br]|ol|wf)|s(?:[tgb]d|[acu]r|[lz]l|os|[hy]p|[dr]g|ek)|z(?:ar|mk|rz)|v(?:uv|ef|nd)|y(?:er|dd)|(?:om|qa)r|wst)\b/i;

$q_check =~ /^$currency_qr$into_qr$currency_qr$/i

$q_escape =~ s/(\%20)en(\%20)/$1in$2/i;
Dates
Enter a date and get the day of week.
my ($ss,$mm,$hh,$day,$month,$year,$zone) = strptime($q);
{
    if ($day && length($day)==4) {
        $year = $day;
        $day = '';
    }
}
if ($month && ($day || $year)) {
  my $tmp_q = $q_escape;
  if ($month && $day) {
  if (!$year) {
    my ($sec,$min,$hour,$mday,$mon,$year2,$wday,$yday,$isdst) =
    localtime(time);
    $year2 = $year2 + 1900;
    $year2 += 1 if $month<$mon || ($month==$mon && $day<$mday);
    $q_escape .= qq(+$year2);
  }
} elsif ($month && $year) {

}
Examples
General Answers
Queries starting/ending with keywords WA answers well.
$query =~ /^(?:(?:number|height|volume|area|circumference|capital) of|mass|radius|density|distance|elevation|population|age|(?:birth|death)\s?(?:year|date)|weight|sun(?:rise|set)|calories|tides?)\s+.*$/i

$query =~ /^.*\s+(?:mass|radius|density|distance|elevation|population|age|(?:birth|death)\s?(?:year|date)|weight|sun(?:rise|set)|calories|tides?)$/i

$query =~ /\bhow many\b.*\b(?:in|on)\b/i
Holidays
Queries matching holiday names (using a lower case and punctuation free version of $query).
my %holidays = map { $_ => undef } (
    'mlk day',
    'mlk',
    'martin luther king day',
    'martin luther king',
    'martin luther kings birthday',
    'presidents day',
    'presidents',
    'washingtons birthday',
    'memorial day',
    'memorial',
    'independence day',
    'independence',
    'labor day',
    'labor',
    'columbus day',
    'columbus',
    'veterans day',
    'veterans',
    'thanksgiving',
    'fathers day',
    'fathers',
    'mothers day',
    'mothers',
    'inauguration day',
    'inauguration',
    'ash wednesday',
    'shrove tuesday',
    'groundhog day',
    'groundhog',
    'saint patricks day',
    'saint patricks',
    'easter',
    'earth day',
    'earth',
    'arbor day',
    'arbor',
    'cinco de mayo',
    'st patricks day',
    'st patricks',
    'flag day',
    'flag',
    'rosh hashanah',
    'yom kippur',
    'passover',
    'halloween',
    'election day',
    'election',
    'hanukkah',
    'christmas',
    'christmas day',
);

exists $holidays{$query}

$q_internal =~ /^!?(?:date\s|day\s|\d{4}\s|\d{2}\s)+(?:of\s|)(.*)\??$/ || $q_internal =~ /^!?(?:(?:when|what) is |(?:how many |)days (?:to |till |until? |for |of )|)(.*?)(?: date| day| \d{4}| \d{2})*\??$/) && exists $holidays{$1}

$tmp_q_escape =~ s/\%27//g;
$tmp_q_escape =~ s/\b(\d{2})\b/my $a = $1;($a<20) ? ($a+=2000) : ($a+=1900)/e;
Location
Queries about weather.
my %location_results = map { $_ => undef } (
    'time',
    'current time',
    'local time',
    'weather',
    'sunrise',
    'sunset',
    'tide',
    'tides',
    );

exists $location_results{$q_check} && $q_check ne 'weather'
Examples
Math
Queries with complicated math (I do simple calcs myself).
$q_check =~ /^\d+\!$/ 

$q_check =~ /^\(?[\d,]+\s?[a-z]+\s*[\^\-\*\(\)\+\/]\s*\d+/i

($q_check =~ /^[xX\^\-\*\(\)\+\/\s\d\,]+$/ && $q !~ /^[\d,]+$/)

($q =~ /(?:\d|\b(?:pi|[xX]|e|one|two|three|four|five|six|seven|eight|nine|ten)\b)/ && $q =~ /(?:\b(?:pi|sqrt|lim|sin|cos|tan|sec|csc|cot|arcsin|arccis|arctan|arccsc|sinh|mod|cosh|tanh|csch|arsinh|arccsch|ln|log|lg|exp|int|choose|integral|days|integrate|sum|derivative|times|divided by|plus|minus|prime|factori[zs]e|sq(?:uare) root)\b|\% of|=)/i)
Examples
Stocks
Ticker quotes.
exists $stocks{lc $query}
Examples
Unicode
Information about characters.
length($q)==1

$q_check =~ /^U\+([\da-f]{4})$/i
Examples
Time
Queries about time.
$query eq 'time'

$query =~ /^(?:current\s|what\s|)(?:(?:local\s?|)time|clock)\s(.*)$/i

$query =~ /\s(?:current\s|what\s|)(?:(?:local\s?|)time|clock)$/i

$query =~ /^(?:what is|what\'s|)\s*(?:today\'?s|)\s*date\s*(?:today|)$/i

$query =~ /^\d{1,2}\:?\d{0,2}[ap]\.?m\.?(?:\s|\Z)/i

$q_escape =~ s/clock/time/i;
$q_escape .= '+time' if $q_escape !~ /time$/i;
Notes
 Copyright (c) 2010 Duck Duck Go, Inc.

 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
 in the Software without restriction, including without limitation the rights
 to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 copies of the Software, and to permit persons to whom the Software is
 furnished to do so, subject to the following conditions:

 The above copyright notice and this permission notice shall be included in
 all copies or substantial portions of the Software.

 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
Here are some areas where I could use some help:
If you think of anything, please let me know.