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  25. <div class="section" lang="en">
    26. 

  26. <div class="titlepage"><div><div><h3 class="title">
    27. 

  27. <a name="boost_regex.background_information.faq"></a><a class="link" href="faq.html" title="FAQ"> FAQ</a>
    28. 

  28. </h3></div></div></div>
    29. 

  29. <p>
    30. 

  30.         <span class="bold"><strong>Q.</strong></span> I can't get regex++ to work with escape
    31. 

  31.         characters, what's going on?
    32. 

  32.       </p>
    33. 

  33. <p>
    34. 

  34.         <span class="bold"><strong>A.</strong></span> If you embed regular expressions in C++
    35. 

  35.         code, then remember that escape characters are processed twice: once by the
    36. 

  36.         C++ compiler, and once by the Boost.Regex expression compiler, so to pass
    37. 

  37.         the regular expression \d+ to Boost.Regex, you need to embed "\d+"
    38. 

  38.         in your code. Likewise to match a literal backslash you will need to embed
    39. 

  39.         "\\" in your code.
    40. 

  40.       </p>
    41. 

  41. <p>
    42. 

  42.         <span class="bold"><strong>Q.</strong></span> No matter what I do regex_match always
    43. 

  43.         returns false, what's going on?
    44. 

  44.       </p>
    45. 

  45. <p>
    46. 

  46.         <span class="bold"><strong>A.</strong></span> The algorithm regex_match only succeeds
    47. 

  47.         if the expression matches <span class="bold"><strong>all</strong></span> of the text,
    48. 

  48.         if you want to <span class="bold"><strong>find</strong></span> a sub-string within
    49. 

  49.         the text that matches the expression then use regex_search instead.
    50. 

  50.       </p>
    51. 

  51. <p>
    52. 

  52.         <span class="bold"><strong>Q.</strong></span> Why does using parenthesis in a POSIX
    53. 

  53.         regular expression change the result of a match?
    54. 

  54.       </p>
    55. 

  55. <p>
    56. 

  56.         <span class="bold"><strong>A.</strong></span> For POSIX (extended and basic) regular
    57. 

  57.         expressions, but not for perl regexes, parentheses don't only mark; they
    58. 

  58.         determine what the best match is as well. When the expression is compiled
    59. 

  59.         as a POSIX basic or extended regex then Boost.Regex follows the POSIX standard
    60. 

  60.         leftmost longest rule for determining what matched. So if there is more than
    61. 

  61.         one possible match after considering the whole expression, it looks next
    62. 

  62.         at the first sub-expression and then the second sub-expression and so on.
    63. 

  63.         So...
    64. 

  64.       </p>
    65. 

  65. <p>
    66. 

  66.         "(0*)([0-9]*)" against "00123" would produce $1 = "00"
    67. 

  67.         $2 = "123"
    68. 

  68.       </p>
    69. 

  69. <p>
    70. 

  70.         where as
    71. 

  71.       </p>
    72. 

  72. <p>
    73. 

  73.         "0*([0-9])*" against "00123" would produce $1 = "00123"
    74. 

  74.       </p>
    75. 

  75. <p>
    76. 

  76.         If you think about it, had $1 only matched the "123", this would
    77. 

  77.         be "less good" than the match "00123" which is both further
    78. 

  78.         to the left and longer. If you want $1 to match only the "123"
    79. 

  79.         part, then you need to use something like:
    80. 

  80.       </p>
    81. 

  81. <p>
    82. 

  82.         "0*([1-9][0-9]*)"
    83. 

  83.       </p>
    84. 

  84. <p>
    85. 

  85.         as the expression.
    86. 

  86.       </p>
    87. 

  87. <p>
    88. 

  88.         <span class="bold"><strong>Q.</strong></span> Why don't character ranges work properly
    89. 

  89.         (POSIX mode only)?
    90. 

  90.       </p>
    91. 

  91. <p>
    92. 

  92.         <span class="bold"><strong>A.</strong></span> The POSIX standard specifies that character
    93. 

  93.         range expressions are locale sensitive - so for example the expression [A-Z]
    94. 

  94.         will match any collating element that collates between 'A' and 'Z'. That
    95. 

  95.         means that for most locales other than "C" or "POSIX",
    96. 

  96.         [A-Z] would match the single character 't' for example, which is not what
    97. 

  97.         most people expect - or at least not what most people have come to expect
    98. 

  98.         from regular expression engines. For this reason, the default behaviour of
    99. 

  99.         Boost.Regex (perl mode) is to turn locale sensitive collation off by not
    100. 

  100.         setting the <code class="computeroutput"><span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">collate</span></code>
    101. 

  101.         compile time flag. However if you set a non-default compile time flag - for
    102. 

  102.         example <code class="computeroutput"><span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">extended</span></code> or <code class="computeroutput"><span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">basic</span></code>,
    103. 

  103.         then locale dependent collation will be enabled, this also applies to the
    104. 

  104.         POSIX API functions which use either <code class="computeroutput"><span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">extended</span></code>
    105. 

  105.         or <code class="computeroutput"><span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">basic</span></code> internally. [Note - when <code class="computeroutput"><span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">nocollate</span></code> in effect, the library behaves
    106. 

  106.         "as if" the LC_COLLATE locale category were always "C",
    107. 

  107.         regardless of what its actually set to - end note].
    108. 

  108.       </p>
    109. 

  109. <p>
    110. 

  110.         <span class="bold"><strong>Q.</strong></span> Why are there no throw specifications
    111. 

  111.         on any of the functions? What exceptions can the library throw?
    112. 

  112.       </p>
    113. 

  113. <p>
    114. 

  114.         <span class="bold"><strong>A.</strong></span> Not all compilers support (or honor)
    115. 

  115.         throw specifications, others support them but with reduced efficiency. Throw
    116. 

  116.         specifications may be added at a later date as compilers begin to handle
    117. 

  117.         this better. The library should throw only three types of exception: [boost::regex_error]
    118. 

  118.         can be thrown by <a class="link" href="../ref/basic_regex.html" title="basic_regex"><code class="computeroutput"><span class="identifier">basic_regex</span></code></a> when compiling a regular
    119. 

  119.         expression, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span></code> can be thrown when a call
    120. 

  120.         to <code class="computeroutput"><span class="identifier">basic_regex</span><span class="special">::</span><span class="identifier">imbue</span></code> tries to open a message catalogue
    121. 

  121.         that doesn't exist, or when a call to <a class="link" href="../ref/regex_search.html" title="regex_search"><code class="computeroutput"><span class="identifier">regex_search</span></code></a> or <a class="link" href="../ref/regex_match.html" title="regex_match"><code class="computeroutput"><span class="identifier">regex_match</span></code></a> results in an "everlasting"
    122. 

  122.         search, or when a call to <code class="computeroutput"><span class="identifier">RegEx</span><span class="special">::</span><span class="identifier">GrepFiles</span></code>
    123. 

  123.         or <code class="computeroutput"><span class="identifier">RegEx</span><span class="special">::</span><span class="identifier">FindFiles</span></code> tries to open a file that cannot
    124. 

  124.         be opened, finally <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">bad_alloc</span></code> can be thrown by just about any
    125. 

  125.         of the functions in this library.
    126. 

  126.       </p>
    127. 

  127. <p>
    128. 

  128.         <span class="bold"><strong>Q.</strong></span> Why can't I use the "convenience"
    129. 

  129.         versions of regex_match / regex_search / regex_grep / regex_format / regex_merge?
    130. 

  130.       </p>
    131. 

  131. <p>
    132. 

  132.         <span class="bold"><strong>A.</strong></span> These versions may or may not be available
    133. 

  133.         depending upon the capabilities of your compiler, the rules determining the
    134. 

  134.         format of these functions are quite complex - and only the versions visible
    135. 

  135.         to a standard compliant compiler are given in the help. To find out what
    136. 

  136.         your compiler supports, run &lt;boost/regex.hpp&gt; through your C++ pre-processor,
    137. 

  137.         and search the output file for the function that you are interested in. Note
    138. 

  138.         however, that very few current compilers still have problems with these overloaded
    139. 

  139.         functions.
    140. 

  140.       </p>
    141. 

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    143. 

  143. <td align="left"></td>
    144. 

  144. <td align="right"><div class="copyright-footer">Copyright &#169; 1998 -2007 John Maddock<p>
    145. 

  145.         Distributed under the Boost Software License, Version 1.0. (See accompanying
    146. 

  146.         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
    147. 

  147.       </p>
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