Back Referencing is capturing what was previously matched and referencing it.

Another use of parenthesis is back referencing. Whenever parenthesis are used the regex engine puts any value derived within parenthesis into a variable. The variables are numbered from the very left parenthesis starting with one 1 to the very right being n.

How they are referenced is platform specific. egrep uses a slash i.e. /1 while perl uses a $ i.e. $1.