e>
<i>back-reference</i> ::= <b>"\"</b> [<b>"1"</b>-<b>"9"</b>]
</pre>
</td></tr>
</table>
<p>
A back reference is a backslash followed by a single non-zero decimal
digit <i>d</i>.  It matches the same sequence of characters
matched by the <i>d</i>th parenthesized subexpression.
</p>

<p>
Back references are not defined for POSIX EREs (for BREs they are),
but many matchers, including TRE, implement back references for both
EREs and BREs.
</p>

<h3>Options</h3>
<a name="options"></a>
<table bgcolor="#e0e0f0"