Regular expressions allow more specific queries then a simple query.

**Examples**

<table>
  <tr>
    <td> compan(y|ies) </td>
    <td> Search for <em>company</em> , <em>companies</em></td>
  </tr>
  <tr>
    <td> (peter|paul) </td>
    <td> Search for <em>peter</em> , <em>paul</em></td>
  </tr>
  <tr>
    <td> bug* </td>
    <td> Search for <em>bug</em> , <em>bugs</em> , <em>bugfix</em></td>
  </tr>
  <tr>
    <td> [Bb]ag </td>
    <td> Search for <em>Bag</em> , <em>bag</em></td>
  </tr>
  <tr>
    <td> b[aiueo]g </td>
    <td> Second letter is a vowel. Matches <em>bag</em> , <em>bug</em> , <em>big</em></td>
  </tr>
  <tr>
    <td> b.g </td>
    <td> Second letter is any letter. Matches also <em>b&amp;amp;g</em></td>
  </tr>
  <tr>
    <td> [a-zA-Z] </td>
    <td> Matches any one letter (not a number and a symbol) </td>
  </tr>
  <tr>
    <td> [^0-9a-zA-Z] </td>
    <td> Matches any symbol (not a number or a letter) </td>
  </tr>
  <tr>
    <td> [A-Z][A-Z]* </td>
    <td> Matches one or more uppercase letters </td>
  </tr>
  <tr>
    <td> [0-9][0-9][0-9]-[0-9][0-9]- <br /> [0-9][0-9][0-9][0-9] </td>
    <td valign="top"> US social security number, e.g. 123-45-6789 </td>
  </tr>
</table>

Here is stuff for our UNIX freaks: <br /> (copied from 'man grep')

         \c   A backslash (\) followed by any special character is  a
              one-character  regular expression that matches the spe-
              cial character itself.  The special characters are:

                   +    `.', `*', `[',  and  `\'  (period,  asterisk,
                        left  square  bracket, and backslash, respec-
                        tively), which  are  always  special,  except
                        when they appear within square brackets ([]).

                   +    `^' (caret or circumflex), which  is  special
                        at the beginning of an entire regular expres-
                        sion, or when it immediately follows the left
                        of a pair of square brackets ([]).

                   +    $ (currency symbol), which is special at  the
                        end of an entire regular expression.

         .    A `.' (period) is a  one-character  regular  expression
              that matches any character except NEWLINE.

         [string]
              A non-empty string of  characters  enclosed  in  square
              brackets  is  a  one-character  regular expression that
              matches any one character in that string.  If, however,
              the  first  character of the string is a `^' (a circum-
              flex or caret), the  one-character  regular  expression
              matches  any character except NEWLINE and the remaining
              characters in the string.  The  `^'  has  this  special
              meaning only if it occurs first in the string.  The `-'
              (minus) may be used to indicate a range of  consecutive
              ASCII  characters;  for example, [0-9] is equivalent to
              [0123456789].  The `-' loses this special meaning if it
              occurs  first (after an initial `^', if any) or last in
              the string.  The `]' (right square  bracket)  does  not
              terminate  such a string when it is the first character
              within it (after an initial  `^',  if  any);  that  is,
              []a-f]  matches either `]' (a right square bracket ) or
              one of the letters a through  f  inclusive.   The  four
              characters  `.', `*', `[', and `\' stand for themselves
              within such a string of characters.

         The following rules may be used to construct regular expres-
         sions:

         *    A one-character regular expression followed by `*'  (an
              asterisk)  is a regular expression that matches zero or
              more occurrences of the one-character  regular  expres-
              sion.   If  there  is  any choice, the longest leftmost
              string that permits a match is chosen.

         ^    A circumflex or caret (^) at the beginning of an entire
              regular  expression  constrains that regular expression
              to match an initial segment of a line.

         $    A currency symbol ($) at the end of an  entire  regular
              expression  constrains that regular expression to match
              a final segment of a line.

         *    A  regular  expression  (not  just   a   one-
              character regular expression) followed by `*'
              (an asterisk) is a  regular  expression  that
              matches  zero or more occurrences of the one-
              character regular expression.   If  there  is
              any  choice, the longest leftmost string that
              permits a match is chosen.

         +    A regular expression followed by `+' (a  plus
              sign)  is  a  regular expression that matches
              one or more occurrences of the  one-character
              regular  expression.  If there is any choice,
              the longest leftmost string  that  permits  a
              match is chosen.

         ?    A regular expression followed by `?' (a ques-
              tion  mark)  is  a  regular  expression  that
              matches zero or one occurrences of  the  one-
              character  regular  expression.   If there is
              any choice, the longest leftmost string  that
              permits a match is chosen.

         |    Alternation:    two    regular    expressions
              separated  by  `|'  or NEWLINE match either a
              match for  the  first  or  a  match  for  the
              second.

         ()   A regular expression enclosed in  parentheses
              matches a match for the regular expression.

         The order of precedence of operators at the same parenthesis
         level  is  `[ ]'  (character  classes),  then  `*'  `+'  `?'
         (closures),then  concatenation,  then  `|'  (alternation)and
         NEWLINE.