1. Variable Variables A variable variable has as its value the name of another variable without $ prefix E.g., if we have $addr, might have a statement $tmp = 'addr'; Can now use the name of the variable variable prefixed by an additional $ in place of variable whose name (w/o $ ) was assigned to it E.g.,, $$tmp = "10 Elm Row"; assigns “10 Elm Row” to $addr echo 'Go to '.$$tmp; outputs Go to followed by value of $addr

2. Can’t replace concatenated string 'Go to'.$$tmp with "Go to $$tmp" The $$tmp inside double quotes is seen as a $ followed by $tmp  Value of $tmp is "addr"

3. Example Have a form with text boxes with the 3 components of an address: street, city, and state. Values of name attributes are addr1, addr2, and add3 In script handling the request, PHP code before HTML tags  sets constant NUMADDRS to 3  copies values of form variables into $addr1, $addr2, and $addr3 PHP code in the body has a for loop ranging $i from 1 to 3  Loop body sets $tmp to "addr$i", i.e., name of variable $addr1, addr2, or $addr3 without $ prefix, depending on $i  Then outputs a string ending with $$tmp, i.e., value of $addr1, $addr2, or $addr3 (depending on $i )

4. <?php define( "NUMADDRS", 3 ); $addr1 = $_POST['addr1']; $addr2 = $_POST['addr2']; $addr3 = $_POST['addr3']; ?> Address Your address is Continued

5. <?php for ( $i=1; $i <= NUMADDRS; $i ++ ) { $tmp = "addr$i"; echo " \n\t".$$tmp; } ?>

6. Use position specifiers to explicitly state which argument values go with which conversions specifiers  To specify the n th argument after the format argument goes in this position, put n$ just after %  Use single quotes (else, e.g., "%1$d" appears to include a variable to be interpolated) Example printf('The age of %2$s is %1$d.', 30, "John"); outputs The age of John is 30. More on printf() (and sprintf() )

7. This allows an argument to be used more than once in the string—e.g., printf('%1$s is %3$d, %2$s is %6$d, %4$s is %6$d, %5$s is %3$d', "Ed", "Al", 39, "Pat", "Sue", 43); outputs Ed is 39, Al is 43, Pat is 43, Sue is 39

8. sprintf() is just like prinft() but returns the formatted string instead of outputting it

9. Option Settings The (modifier) option can be changed from within the pattern by a sequence of option letters (modifiers) enclosed between (? and ) The new option is in effect from that point to the end of the pattern or enclosing subpattern E.g., "/ab(?is)c.d/" match "abc\nd" as well as "abC\nd" Turn off an option by having it to the right of "-" —e.g., "/ab(?is)c.d(?-i)ef/" Additional on PHP Regular Expressions

10. If any option settings are required at the start of a non-capturing subpattern, the option letters may appear between the ? and the :  E.g., the following have the same effect (?i:saturday|sunday) (?:(?i)saturday|sunday)

11. \b, as in JavaScript, matches a word boundary \B matches where there is no word boundary  E.g., "/\Band\B/" matches "hands" but not "one and two" \Z and \z, unlike $, match only the very end of the subject string even when the m modifier is used  \Z matches before a newline that is the last character of the string as well as at the end of the string  \z matches only at the end Simple (Point-based) Assertions

12. \A, unlike ^, matches only the very start of the subject string even when the m modifier is used If preg_match() is given a 4 th argument, it’s the offset into the subject string where the search for a match starts  E.g., preg_match("/ab/", "ababab", $m, 2) matches the 2 nd " ab " in the " ababab " \G is true only when the current matching position is at the start point of the match, as specified by the offset argument  Differs from \A when the offset is non-zero  E.g., preg_match("/\Gab/", "ababab", $m, 1) fails

13. Besides, e.g., \1, can also use \g1 or \g{1}  The longer forms avoid ambiguity in some cases \g with a negative number is a relative reference  E.g., "/(foo)(bar)\g{-1}/" matches "foobarbar" and "/(foo)(bar)\g{-2}/" matches "foobarfoo" More on Back References

14. Can name a subpattern using the syntax (?P pattern)  Alternatively, (? pattern) or (?'name'pattern)  The subpattern is then indexed in the matches array by its normal numeric position and also by name  For backreference, can use (?P=name), \k, \k'name', \k{name}, or \g{name} Example $pat = "/[a-z]+(? \d+)[a-z]+/"; $str = "abcd123efg"; preg_match($pat, $str, $m); echo $m['num']. " ". $m[1]; outputs 123 Named Subpatterns

15. Sometimes we need alternative capturing subpatterns  Each gets its own backreference number even though only 1 ever matches  The (?| syntax allows duplicate numbers  E.g., matching "/(?:(Sat)ur|(Sun))day/" against " Sunday ", " Sun " is stored in backreference 2 and backreference 1 is empty Using "/(?|(Sat)ur|(Sun))day/" both "Sat" and "Sun" can get stored in backreference 1 Alternative Capturing Subpatterns

16. An assertion subpattern is matched in the normal way but doesn’t change the current matching position Lookaheads Lookaheads can be negative or positive A negative look ahead is denoted by ?!  Check that a given pattern isn’t following a match we’re making Examples preg_match("/white(?!house)/", "His whitehouse");  Fails: the pattern says "white" shouldn’t be immediately followed by "house" preg_match("/white(?!house)/", "His white house");  Succeeds: cf. the space between "white" and "house" Assertions

17. A positive lookahead is denoted by ?=  Check that a pattern follows a match we’re making  E.g., "/pay(?=day)/" Lookbehinds A positive lookbehind is denoted by ?<=  Check that a pattern precedes a match we’re making  E.g., " /(?<=pay)day/ " A negative lookbehind is denoted by ?<!  Check that a pattern doesn’t precede a match we’re making  E.g., " /(?<!white)house/ "

18. Can cause matching to obey a subpattern conditionally or to choose between two alternative subpatterns The 2 forms are (?(condition)yes-pattern) (?(condition)yes-pattern|no-pattern)  If the condition is satisfied, yes-pattern is used; else no-pattern (if present) 2 kinds of condition If the text between the parentheses consists of a digit, then the condition is satisfied if the capturing subpattern of that number has previously matched Conditional Subpatterns

19. Example "/(\()\d+(?(1)\))/"  Matches a string of digits  If the digits are preceded by a (, look for a closing ) If the condition isn’t a digit, it must be (2 nd kind of condition) an assertion: positive or negative lookahead or lookbehind Example "/((?=a)abc|zyx)/"  If the next character is an a, look for abc, else look for zyx

20. Recall: preg_replace( pattern, replacement, subject ) returns string subject with all non-overlapping substrings matching regex pattern replaced by the string replacement If subject is an array, then preg_replace() does the replacement on every element of the array and returns an array with the result (ordered as per the original) Example $strs = array("2013-10-25", "2012-1-13"); $strs1 = preg_replace("/(\d+)-(\d+)-(\d+)/", "$2/$3/$1", $strs); foreach ( $strs1 as $s ) echo $s. " "; outputs 10/25/2013 1/13/2012 preg_replace() with Array Arguments

21. pattern and replacement can be arrays of the same length  When an element of pattern matches, it’s replaced by the corresponding element in replacement Example Given $pats = array("/([A-Z][a-z]*)\s+([A-Z][a-z]*)/", "/(\d+)-(\d+)-(\d+)/"); $repls = array("$2, $1", "$2/$3/$1"); executing preg_replace($pats, $repls, "John Doe: 2012-1-13. Sue Smith: 2011-11-24"); returns Doe, John: 1/13/2012. Smith, Sue: 11/24/2011

22. The modifier e is specific to preg_replace()  When pattern has an e modifier, preg_replace() does normal substitution of backreferences in the replacement string, evaluates the replacement string as PHP code, and uses the result for replacing the search string Example—note that the replacement has to be double-quoted as it gets evaluated preg_replace("/ ([a-z])([a-z]*)/e", "' '.strtoupper('$1').'$2'", " john doe, mary smith") returns the string " John Doe, Mary Smith" Replacement Modifier e