Sep 4 2019

Rex – Splunk Documentation #splunk #rex

Search Reference


Use this command to either extract fields using regular expression named groups, or replace or substitute characters in a field using sed expressions.

The rex command matches the value of the specified field against the unanchored regular expression and extracts the named groups into fields of the corresponding names. If a field is not specified, the regular expression is applied to the _raw field. Note: Running rex against the _raw field might have a performance impact.

When mode=sed, the given sed expression used to replace or substitute characters is applied to the value of the chosen field. If a field is not specified, the sed expression is applied to _raw. This sed-syntax is also used to mask sensitive data at index-time . Read about using sed to anonymize data in the Getting Data In Manual.

Use the rex command for search-timefield extraction or string replacement and character substitution.


rex [field= field ] ( regex-expression [max_match= int ] [offset_field= string ] ) | (mode=sed sed-expression )

Required arguments

regex-expression Syntax: ” string ” Description: The PCRE regular expression that defines the information to match and extract from the specified field. Quotation marks are required. mode Syntax: mode=sed Description: Specify to indicate that you are using a sed (UNIX stream editor) expression. sed-expression Syntax: ” string ” Description: When mode=sed, specify whether to replace strings (s) or substitute characters (y) in the matching regular expression. No other sed commands are implemented. Quotation marks are required. Sed mode supports the following flags: global (g) and Nth occurrence (N), where N is a number that is the character location in the string.

Optional arguments

field Syntax: field= field Description: The field that you want to extract information from. Default: _raw max_match Syntax: max_match= int Description: Controls the number of times the regex is matched. If greater than 1, the resulting fields are multivalued fields. Default: 1, use 0 to mean unlimited. offset_field Syntax: offset_field= string Description: If provided, a field is created with the name specified by string . This value of the field has the endpoints of the match in terms of zero-offset characters into the matched field. For example, if the rex expression is “(? tenchars .<10>)”, this matches the first ten characters of the field, and the offset_field contents is “0-9”. Default: unset

Sed expression

When using the rex command in sed mode, you have two options: replace (s) or character substitution (y).

The syntax for using sed to replace (s) text in your data is: “s/ regex / replacement / flags “

  • regex is a PCRE regular expression, which can include capturing groups.
  • replacement is a string to replace the regex match. Use n for backreferences, where “n” is a single digit.
  • flags can be either: g to replace all matches, or a number to replace a specified match.

The syntax for using sed to substitute characters is: “y/ string1 / string2 /”

  • This substitutes the characters that match string1 with the characters in string2 .


Splunk SPL uses perl-compatible regular expressions (PCRE).

When you use regular expressions in searches, you need to be aware of how characters such as pipe ( | ) and backslash ( ) are handled. See SPL and regular expressions in the Search Manual.

For general information about regular expressions, see Splunk Enterprise regular expressions in the Knowledge Manager Manual.


Example 1:

Extract “from” and “to” fields using regular expressions. If a raw event contains “From: Susan To: Bob”, then from=Susan and to=Bob.

| rex field=_raw From: (? from .*) To: (? to .*)

Example 2:

Extract “user”, “app” and “SavedSearchName” from a field called “savedsearch_id” in scheduler.log events. If savedsearch_id=bob;search;my_saved_search then user=bob. app=search and SavedSearchName=my_saved_search

| rex field=savedsearch_id (? user w+);(? app w+);(? SavedSearchName w+)

Example 3:

Use sed syntax to match the regex to a series of numbers and replace them with an anonymized string.

| rex field=ccnumber mode=sed s/(d<4>-)<3>/XXXX-XXXX-XXXX-/g

Example 4:

Display IP address and ports of potential attackers.

sourcetype=linux_secure port failed password | rex s+(? ports port d+) | top src_ip ports showperc=0

This search used rex to extract the port field and values. Then, it displays a table of the top source IP addresses (src_ip) and ports the returned with the search for potential attackers.

See also

Written by CREDIT

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