CSE134A LECTURE NOTES

ANNOUNCEMENTS

Remember that the midterm is next Wednesday, October 31.  Here are the Spring 2001 midterm and final examination.  Sample solutions are not available currently.  The midterm will count for 1/6 of your overall grade.
 
 

USING REGULAR EXPRESSIONS

The PHP function ereg($pattern,$input) returns TRUE if the pattern is found in the string.  Remember to use ^ and $ if you want the pattern to match only if it matches the whole string.  The function eregi() is similar except it ignores upper case/lower case differences.

The function split($pattern,$input) returns an array of strings that is the result of dividing up its second argument into pieces, using matches to the first argument as a delimiter.  For example:

list ($month, $day, $year) = split ('[/.-]', $date);

If the delimiter is not found or the delimiter is empty, then the first element of the array (with subscript 0) gets the whole input.  If the delimiter is repeated consecutively, the array will include null items.

An optional third argument says how many items to return.  The last item then contains the whole remainder of the string.  This is useful for writing  before() and after() functions that take a specific part of a web page   These functions are useful for information extraction, for example:

function after($pattern, $text) {
    if ($pattern == "") return $text;
    $s = split($pattern, $text, 2);
    return $s[1];
}

HOW DO REGULAR EXPRESSIONS MATCH?

Leftmost, longest behavior means that to match the first string delimited by single quotes you must write the pattern '[^']*'  This pattern explicitly says that quote characters are not allowed inside the match.

Note that the top priority for the match found is "leftmost."  "Longest" is only the second priority.
 
 

EFFICIENCY WITH REGULAR EXPRESSIONS

Avoid especially expressions that match multiple ways.  For example do not write .*<big><b>.*</b></big>.*  This is bad for several reasons.

• One, the .* at the beginning and end are unnecessary.  Don't add ^ and $, instead delete the .*
• Two, if there are multiple <big><b> regions, this will match from the beginning of the first to the end of the last.

    $text = after("<big><b>",$text);
    $title = before("</b></big>",$text);

When possible, use plain strings instead of regular expressions.  The explode() function has the same effect as split(), but the delimiter is an ordinary string, not a regular expression.  Therefore explode() is much more efficient.

For the new project and in general, it is important that you adopt a well-organized and efficient approach for doing information extraction.  Do not just use regular expressions developed by trial and error.  One part of your report should be an explanation of your  approach, which should be as clear and simple as possible.  In the report, describe the capabilities and limitations of your strategy.  Which changes in the data sources could you handle, and which changes would break your strategy?
 
 

EXTRACTING PARTS OF A STRING

If ereg() finds any matches at all, then $regs is filled with exactly ten elements, even though more or fewer than ten parenthesized substrings may actually match.  If no matches are found, then $regs is not altered.

For example, to convert from an ISO date to a U.S. style date:

if (ereg ("([0-9]{4})-([0-9]{1,2})-([0-9]{1,2})", $date, $regs)) {
    echo "$regs[3].$regs[2].$regs[1]";

    }

else { echo "Invalid date format: $date"; } 

 

 

REPLACING PARTS OF A STRING

For example, suppose we want to separate all the words in a string by commas:

ereg_replace("[ \n\r\t]+", ",", trim($str));

$string=ereg_replace("([a-z])([A-Z])", "\\1xxx\\2" , "FieldNamePlus");         

FieldxxxNamexxxPlus