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The parentheses in the regular expression (see the previous section) are used to group items together into a sub-expression and to mark what generated the match. Each sub-expression can be then referenced within the “Format” text box using the $N expression (see below).
The text box defines the actual value of the field based on text matched by the regular expression typed in the “Regex” text box. You can type anything in the text box and use the following perl-like expressions to use regular expression matches. The expressions will expand to a particular matched sub-expression:
$'
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Expands to text from the end match to the end of the message.
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$&
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Expands to text matched by the entire regular expression.
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$N
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Expands to text matched by N-th sub-expression, for instance $1 expands to the text matched by the first sub-expression.
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