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SPL = Splunk Processing Language

host="<host>"
source="<source>"
sourcetype="<sourcetype>"

* can be used as a wildcard
| used to chain filtering / commands / searches

Full Search Reference: http://docs.splunk.com/Documentation/Splunk/7.0.1/SearchReference/WhatsInThisManual

dedup <field>
    Deduplicate by <field> values
head <number>
    Show first <number> results
tail <number>
    Show last <number> results
reverse
    Reverse results
search <search>
    Search results    
sort <field>
    Sort by <field> ascending
sort -<field>
    Sort by <field> descending
top [<count>] <field>
    Show event counts by <field>
timechart <field> [BY <field>]
    Show agrregate graph

eval can be used in order to create variables and do calculations
    e.g. eval var=10/5 | search secs
    Allows you to add a generated / calculated field

stats can be used for generating statistics based on the results (which will be shown in the statistics tab)
  Lots of funcs here http://docs.splunk.com/Documentation/Splunk/7.0.1/SearchReference/CommonStatsFunctions
  e.g. distinct_count(<field>)

rex can be used for regular expressions
    rex [field=<field>] ( <regex-expression> [max_match=<int>] [offset_field=<string>] ) | (mode=sed <sed-expression>)
    e.g. rex mode="sed" field=uri "s/=[\d\w]+\&/=NULLED\&/g"
    Note: Ampersand represents full captured string in replace (so escape it)