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We have defined \1, \2, \3 .. as variables set by parentheses and used internally to a match. These variables are available outside the regular expression operation with conventional PERL names $1 $2 $3 etc. Use latter even in substitution.
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In string matched in part by a regular expression, we can identify three parts
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$` is variable holding part of string BEFORE matched part
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$& is variable holding part of string matched by regular expression
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$' is variable holding part of string AFTER matched part
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So string is concatenation $` . $& . $'
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