Capture Group References

In Wrangle transformations that support use of patterns, you may need to specify capture groups. A capture group is a pattern that describes a set of one or more characters that constitute a match. These matches can be programmatically referenced in replacement values.

  • These patterns are described using regular expression syntax. Cloud Dataprep by TRIFACTA INC. implements a version of regular expressions based off of RE2 and PCRE regular expressions.

Basic Capture Groups

Example 1

replace col:* on:`{start}(%+) ` with:'First Word\:$1'

Elements of the matching pattern (on:):

ReferenceDescription
{start}

A Cloud Dataprep pattern reference to the start of the tested value.

(%+)

Matches on one or more characters of any time.

NOTE: The parentheses indicate that this set of characters is a capture group.

Last character in the matching pattern is an empty space.

Matches: First set of any characters in the tested value up to the first empty space (the first word), across all columns of the dataset.

Replaced with: The text value First Word: followed by a reference to the first capture group ($1), which returns the first word found in the tested value.

Example 2

The previous example works fine, as long as there is a space in the tested value to identify the end of the first word. If there is one and only word in the tested value, then you must amend the on: parameter value with the following:

replace col:* on:`{start}(%+) ( |{end})` with:'First Word\:$1'

In this case, the second capture group features two elements:

ReferenceDescription
first character in the second capture group is an empty space.
|Logical OR, which means that the capture group matches on either the empty space or the following value, which is a reference to the end of the tested value.
{end}

A Cloud Dataprep pattern reference to the end of the tested value.

Example 3

replace col:* on:`{start}(%+) (%+)( |{end})` with:'Second Word\:$2'

Matches: The on: pattern has been augmented to include the second word in the tested value, across all columns of the dataset.

Replaced with: The text value Second Word: followed by a reference to the second capture group ($2), which returns the second word found in the tested value.

Dollar sign in replace transform

The dollar sign ($) is used as a form of escape character in the with parameter of the replace transform. This pattern identifies the replacement string.

In the table below, you can review how these replacement patterns are supported.

PatternDescription
$$Inserts a $ in the replacement value.
$n or
$nn
For non-negative digits n, this pattern inserts the nth parameterized sub-match string, provided that the first argument was a regex object.

Examples

In the following example, the MyColumn column contains the value foobar in all rows.

source valuereplace transformreplacement
foobar
replace col:MyColumn with:'$$f' on:'f'
$foobar
foobar
replace col:MyColumn with:'$2' on:`(f)(o)o(b)ar`

Note that the on parameter is a Cloud Dataprep pattern.

o

Positive and Negative Lookaheads

In regular expressions, you can use positive and negative lookahead capture groups to capture content that is conditionally followed or not followed by a specified capture group.

TypeExample expression
Positive lookahead
/q(?u)/
Capture the letter q only when it is followed by the letter u. Letter u is not captured.
Negative lookahead
/q(?!u)/
Capture the letter q when it is not followed by the letter u. Letter u is not captured.
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