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rex

Description

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-time field extraction or string replacement and character substitution.

Syntax

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>.

Usage

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.

Examples

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

extract, kvform, multikv, regex, spath, xmlkv,

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This documentation applies to the following versions of Splunk Cloud: 6.5.0, 6.5.1, 6.5.1612, 6.6.0, 6.6.1, 6.6.3


Comments

Woodcock
Thank you for the suggestion. I will pass your request along to the development team.

Lstewart splunk, Splunker
March 17, 2017

We really need this command to support the `_KEY_<string>`, `_VAL_<string>` feature:
https://docs.splunk.com/Documentation/Splunk/6.5.2/Admin/Transformsconf

Woodcock
March 16, 2017

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