[/ 
  Copyright 2006-2007 John Maddock.
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]


[section:boost_format_syntax Boost-Extended Format String Syntax]

Boost-Extended format strings treat all characters as literals except for 
'$', '\\', '(', ')', '?', and ':'.

[h4 Grouping]

The characters '(' and ')' perform lexical grouping, so use \\( and \\) if you 
want a to output literal parenthesis.

[h4 Conditionals]

The character '?' begins a conditional expression, the general form is:

?Ntrue-expression:false-expression

where N is decimal digit.

If sub-expression N was matched, then true-expression is evaluated and sent to 
output, otherwise false-expression is evaluated and sent to output.

You will normally need to surround a conditional-expression with parenthesis in 
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 
should be sent to output as follows:

[table
[[Placeholder][Meaning]]
[[$&][Outputs what matched the whole expression.]]
[[$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.

[h4 Escape Sequences]

An escape character followed by any character x, outputs that character unless 
x is one of the escape sequences shown below.

[table
[[Escape][Meaning]]
[[\\a][Outputs the bell character: '\\a'.]]
[[\\e][Outputs the ANSI escape character (code point 27).]]
[[\\f][Outputs a form feed character: '\\f']]
[[\\n][Outputs a newline character: '\\n'.]]
[[\\r][Outputs a carriage return character: '\\r'.]]
[[\\t][Outputs a tab character: '\\t'.]]
[[\\v][Outputs a vertical tab character: '\\v'.]]
[[\\xDD][Outputs the character whose hexadecimal code point is 0xDD]]
[[\\x{DDDD}][Outputs the character whose hexadecimal code point is 0xDDDDD]]
[[\\cX][Outputs the ANSI escape sequence "escape-X".]]
[[\\D][If D is a decimal digit in the range 1-9, then outputs the text that matched sub-expression D.]]
[[\\l][Causes the next character to be outputted, to be output in lower case.]]
[[\\u][Causes the next character to be outputted, to be output in upper case.]]
[[\\L][Causes all subsequent characters to be output in lower case, until a \\E is found.]]
[[\\U][Causes all subsequent characters to be output in upper case, until a \\E is found.]]
[[\\E][Terminates a \\L or \\U sequence.]]
]

[endsect]