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<b class="current">Regular Expressions</b>
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<div><h2 id="a0000000033">6.1.2 Regular Expressions</h2>
<p>String variables can be modified using the search-and-replace string operator<a name="a0000000570" id="a0000000570"></a><a href="#a0000000571" class="footnote"><sup class="footnotemark">1</sup></a>, =<img src="images/img-0028.png" alt="$\sim $" style="vertical-align:2px;
width:12px;
height:5px" class="math gen" /><a name="a0000000572" id="a0000000572"></a>, which takes a regular expression with a syntax similar to that expected by the shell command <tt class="tt">sed</tt><a name="a0000000573" id="a0000000573"></a> and applies it to the relevant string variable.<a href="#a0000000574" class="footnote"><sup class="footnotemark">2</sup></a><a name="a0000000575" id="a0000000575"></a> In the following example, the first instance of the letter <tt class="tt">s</tt> in the string variable <tt class="tt">twister</tt> is replaced with the letters <tt class="tt">th</tt>: </p><p> <tt class="tt">pyxplot> <b class="bf">twister="seven silver soda syphons"</b></tt><br /><tt class="tt">pyxplot> <b class="bf">twister =<img src="images/img-0028.png" alt="$\sim $" style="vertical-align:2px;
width:12px;
height:5px" class="math gen" /> s/s/th/</b></tt><br /><tt class="tt">pyxplot> <b class="bf">print twister</b></tt><br /><tt class="tt">theven silver soda syphons</tt> </p><p>Note that only the <tt class="tt">s</tt> (substitute) command of <tt class="tt">sed</tt> is implemented in PyXPlot. Any character can be used in place of the <tt class="tt">/</tt> characters in the above example, for example: </p><pre>
twister =~ s's'th'
</pre><p>Flags can be passed, as in <tt class="tt">sed</tt> or <tt class="tt">perl</tt>, to modify the precise behaviour of the regular expression. In the following example the <tt class="tt">g</tt> flag is used to perform a global search-and-replace of all instances of the letter <tt class="tt">s</tt> with the letters <tt class="tt">th</tt>: </p><p> <tt class="tt">pyxplot> <b class="bf">twister="seven silver soda syphons"</b></tt><br /><tt class="tt">pyxplot> <b class="bf">twister =<img src="images/img-0028.png" alt="$\sim $" style="vertical-align:2px;
width:12px;
height:5px" class="math gen" /> s/s/th/g</b></tt><br /><tt class="tt">pyxplot> <b class="bf">print twister</b></tt><br /><tt class="tt">theven thilver thoda thyphonth</tt> </p><p>Table <a href="sect0032.html#tab:re_flags">6.2</a> lists all of the regular expression flags recognised by the =<img src="images/img-0028.png" alt="$\sim $" style="vertical-align:2px;
width:12px;
height:5px" class="math gen" /> operator. </p><div id="tab:re_flags" class="table"><p><small class="footnotesize"><table cellspacing="0" class="tabular">
<tr>
<td style="border-top-style:solid; text-align:left; border-top-color:black; border-top-width:1px; border-left:1px solid black"><p> <tt class="tt">g</tt> </p></td>
<td style="border-top-style:solid; text-align:left; border-top-color:black; border-top-width:1px; border-right:1px solid black"><p> Replace <i class="it">all</i> matches of the pattern; by default, only the first match is replaced. </p></td>
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<td style="text-align:left; border-left:1px solid black"><p><tt class="tt">i</tt> </p></td>
<td style="text-align:left; border-right:1px solid black"><p> Perform case-insensitive matching, such that expressions like <tt class="tt">[A-Z]</tt> will match lowercase letters, too. </p></td>
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<td style="text-align:left; border-left:1px solid black"><p><tt class="tt">l</tt> </p></td>
<td style="text-align:left; border-right:1px solid black"><p> Make <tt class="tt"><img src="images/img-0006.png" alt="$\backslash $" style="vertical-align:-5px;
width:7px;
height:18px" class="math gen" />w</tt>, <tt class="tt"><img src="images/img-0006.png" alt="$\backslash $" style="vertical-align:-5px;
width:7px;
height:18px" class="math gen" />W</tt>, <tt class="tt"><img src="images/img-0006.png" alt="$\backslash $" style="vertical-align:-5px;
width:7px;
height:18px" class="math gen" />b</tt>, <tt class="tt"><img src="images/img-0006.png" alt="$\backslash $" style="vertical-align:-5px;
width:7px;
height:18px" class="math gen" />B</tt>, <tt class="tt"><img src="images/img-0006.png" alt="$\backslash $" style="vertical-align:-5px;
width:7px;
height:18px" class="math gen" />s</tt> and <tt class="tt"><img src="images/img-0006.png" alt="$\backslash $" style="vertical-align:-5px;
width:7px;
height:18px" class="math gen" />S</tt> dependent on the current locale. </p></td>
</tr><tr>
<td style="text-align:left; border-left:1px solid black"><p><tt class="tt">m</tt> </p></td>
<td style="text-align:left; border-right:1px solid black"><p> When specified, the pattern character <tt class="tt"><img src="images/img-0024.png" alt="\^{}" style="vertical-align:9px; width:5px; height:4px" class="accent gen" /></tt> matches the beginning of the string and the beginning of each line immediately following each newline. The pattern character <tt class="tt">$</tt> matches at the end of the string and the end of each line immediately preceding each newline. By default, <tt class="tt"><img src="images/img-0024.png" alt="\^{}" style="vertical-align:9px; width:5px; height:4px" class="accent gen" /></tt> matches only the beginning of the string, and <tt class="tt">$</tt> only the end of the string and immediately before the newline, if present, at the end of the string. </p></td>
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<td style="text-align:left; border-left:1px solid black"><p><tt class="tt">s</tt> </p></td>
<td style="text-align:left; border-right:1px solid black"><p> Make the <tt class="tt">.</tt> special character match any character at all, including a newline; without this flag, <tt class="tt">.</tt> will match anything except a newline. </p></td>
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<td style="text-align:left; border-left:1px solid black"><p><tt class="tt">u</tt> </p></td>
<td style="text-align:left; border-right:1px solid black"><p> Make <tt class="tt"><img src="images/img-0006.png" alt="$\backslash $" style="vertical-align:-5px;
width:7px;
height:18px" class="math gen" />w</tt>, <tt class="tt"><img src="images/img-0006.png" alt="$\backslash $" style="vertical-align:-5px;
width:7px;
height:18px" class="math gen" />W</tt>, <tt class="tt"><img src="images/img-0006.png" alt="$\backslash $" style="vertical-align:-5px;
width:7px;
height:18px" class="math gen" />b</tt>, <tt class="tt"><img src="images/img-0006.png" alt="$\backslash $" style="vertical-align:-5px;
width:7px;
height:18px" class="math gen" />B</tt>, <tt class="tt"><img src="images/img-0006.png" alt="$\backslash $" style="vertical-align:-5px;
width:7px;
height:18px" class="math gen" />s</tt> and <tt class="tt"><img src="images/img-0006.png" alt="$\backslash $" style="vertical-align:-5px;
width:7px;
height:18px" class="math gen" />S</tt> dependent on the Unicode character properties database. </p></td>
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<td style="border-bottom-color:black; border-bottom-width:1px; text-align:left; border-bottom-style:solid; border-left:1px solid black"><p><tt class="tt">x</tt> </p></td>
<td style="border-bottom-color:black; border-bottom-width:1px; text-align:left; border-bottom-style:solid; border-right:1px solid black"><p> This flag allows the user to write regular expressions that look nicer. Whitespace within the pattern is ignored, except when in a character class or preceded by an un-escaped backslash. When a line contains a <tt class="tt">#</tt>, neither in a character class nor preceded by an un-escaped backslash, all characters from the left-most such <tt class="tt">#</tt> through to the end of the line are ignored. </p></td>
</tr>
</table></small> </p><div class="caption"><b>Table 6.2</b>: <span>A list of the flags accepted by the =<img src="images/img-0028.png" alt="$\sim $" style="vertical-align:2px;
width:12px;
height:5px" class="math gen" /> operator. Most are rarely used, but the <tt class="tt">g</tt> flag is very useful.</span></div></div></div>
<div id="footnotes">
<p><b>Footnotes</b></p>
<ol>
<li id="a0000000571">Programmers with experience of <tt class="tt">perl</tt> will recognise this syntax.</li><li id="a0000000574">Regular expression syntax is a massive subject, and is beyond the scope of this manual. The official GNU documentation for the <tt class="tt">sed</tt> command is heavy reading, but there are many more accessible tutorials on the web.</li>
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