PHP's ereg functions are deprecated, and you should now be moving to the preg functions. See [here]. As you're switching your PHP code from ereg to preg regular expressions, you'll be adding delimiters to the main regular expression and providing the ability to add modifiers onto the end - a series of single letters each of which alters a behaviour throughout the regular expression.

Perl users may be familiar with the following, all of which are also available in PHP's preg functions:

i    ignore case
m    match ^ and $ at embedded new lines as well as very start and end of string
s    matches anything (by default matches anything except \n)
x    white spaces and comments (# to line end) are treated as comments
e    preg_replace only. evaluate OUTPUT string as PHP code and substitute the result

In addition, PHP also supports the following modifiers:

A    assume ^ anchor at start
D    assume $ anchor at end
S    Study; do more initial analysis for faster matching
U    default counts to sparse (ungreedy). Extra ? becomes greedy (i.e. invert default)
X    unknown \letter combination's are to throw an error not be ignored
J    Allow duplicate names for sub-patterns
u    treat patterns as UTF-8 rather than ASCII

However, these two perl modifiers are NOT available in PHP:

g    global (use count instead on preg_replace, or use preg_match_all rather than preg_match)
o    once only. An efficiency thing in Perl. No code change needed in PHP

The full technical reference that describes each of these in detail (but currently lacks my summary table!) is [here]. We cover regular expressions in PHP on both our earning to program in PHP and PHP Programming courses. They are covered in more depth on our PHP techniques course - a second level course which helps delegates who already know PHP make the best of the language, and we also offer a complete Regular Expressions day.
(written 2011-11-12, updated 2011-11-27)

 H107 - String Handling in PHP
  [3534] Learning to program in PHP - Regular Expression and Associative Array examples - (2011-12-01)
  [3515] PHP - moving from ereg to preg for regular expressions - (2011-11-11)
  [3424] Divide 10000 by 17. Do you get 588.235294117647, 588.24 or 588? - Ruby and PHP - (2011-09-08)
  [3020] Handling (expanding) tabs in PHP - (2010-10-29)
  [2629] Curly braces within double quoted strings in PHP - (2010-02-09)
  [2238] Handling nasty characters - Perl, PHP, Python, Tcl, Lua - (2009-06-14)
  [2165] Making Regular Expressions easy to read and maintain - (2009-05-10)
  [2046] Finding variations on a surname - (2009-02-17)
  [1799] Regular Expressions in PHP - (2008-09-16)
  [1613] Regular expression for 6 digits OR 25 digits - (2008-04-16)
  [1603] Do not SHOUT and do not whisper - (2008-04-06)
  [1533] Short and sweet and sticky - PHP form input - (2008-02-06)
  [1372] A taster PHP expression ... - (2007-09-30)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1195] Regular Express Primer - (2007-05-20)
  [1058] PHP Regular expression to extrtact link and text - (2007-01-31)
  [1008] Date conversion - PHP - (2006-12-26)
  [728] Looking ahead and behind in a Regular Expression - (2006-05-22)
  [716] Evaluating arithmetic expressions in configuration files - (2006-05-10)
  [642] How similar are two words - (2006-03-11)
  [608] Don't expose your regular expressions - (2006-02-15)
  [589] Robust PHP user inputs - (2006-02-03)
  [574] PHP - dividing a string up into pieces - (2006-01-23)
  [560] The fencepost problem - (2006-01-10)
  [558] Converting between acres and hectares - (2006-01-08)
  [493] Running a Perl script within a PHP page - (2005-11-12)
  [463] Splitting the difference - (2005-10-13)
  [422] PHP Magic Quotes - (2005-08-22)
  [337] the array returned by preg_match_all - (2005-06-06)
  [54] PHP and natural sorting - (2004-09-19)
  [31] Here documents - (2004-08-28)

Q803 - Object Orientation and General technical topics - Regular Expressions - Extra Elements
  [3650] Possessive Regular Expression Matching - Perl, Objective C and some other languages - (2012-03-12)
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)
  [3089] Python regular expressions - repeating, splitting, lookahead and lookbehind - (2010-12-17)
  [2909] Be gentle rather than macho ... regular expression techniques - (2010-08-08)
  [1860] Seven new intermediate Perl examples - (2008-10-30)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1601] Replacing the last comma with an and - (2008-04-04)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)

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