Add support for named and/or subexpressions with indexes > 99 in conditional format expressions.

[SVN r52873]

doc/format_boost_syntax.qbk

@@ -33,6 +33,15 @@ order to prevent ambiguities.
 For example, the format string "(?1foo:bar)" will replace each match found with "foo" if
 the sub-expression $1 was matched, and with "bar" otherwise.
 
+For sub-expressions with an index greater than 9, or for access to named sub-expressions use:
+
+?{INDEX}true-expression:false-expression
+
+or
+
+?{NAME}true-expression:false-expression
+
+
 [h4 Placeholder Sequences]
 
 Placeholder sequences specify that some part of what matched the regular expression 
@@ -41,12 +50,24 @@ should be sent to output as follows:
 [table
 [[Placeholder][Meaning]]
 [[$&][Outputs what matched the whole expression.]]
-[[$`][Outputs the text between the end of the last match found (or the 
-            start of the text if no previous match was found), and the start 
-            of the current match.]]
+[[$MATCH][As $&]]
+[[${^MATCH}][As $&]]
+[[$\`][Outputs the text between the end of the last match found (or the 
+      start of the text if no previous match was found), and the start 
+      of the current match.]]
+[[$PREMATCH][As $\`]]
+[[${^PREMATCH}][As $\`]]
 [[$'][Outputs all the text following the end of the current match.]]
+[[$POSTMATCH][As $']]
+[[${^POSTMATCH}][As $']]
+[[$+][Outputs what matched the last marked sub-expression in the regular expression.]]
+[[$LAST_PAREN_MATCH][As $+]]
+[[$LAST_SUBMATCH_RESULT][Outputs what matched the last sub-expression to be actually matched.]]
+[[$^N][As $LAST_SUBMATCH_RESULT]]
 [[$$][Outputs a literal '$']]
 [[$n][Outputs what matched the n'th sub-expression.]]
+[[${n}][Outputs what matched the n'th sub-expression.]]
+[[$+{NAME}][Outputs whatever matched the sub-expression named "NAME".]]
 ]
 
 Any $-placeholder sequence not listed above, results in '$' being treated as a literal.