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Last updated: Thu, 21 Aug 2003

XCI. Regular Expression Functions (POSIX Extended)


Tip: PHP also supports regular expressions using a Perl-compatible syntax using the PCRE functions. Those functions support non-greedy matching, assertions, conditional subpatterns, and a number of other features not supported by the POSIX-extended regular expression syntax.


These regular expression functions are not binary-safe. The PCRE functions are.

Regular expressions are used for complex string manipulation in PHP. The functions that support regular expressions are:

These functions all take a regular expression string as their first argument. PHP uses the POSIX extended regular expressions as defined by POSIX 1003.2. For a full description of POSIX regular expressions see the regex man pages included in the regex directory in the PHP distribution. It's in manpage format, so you'll want to do something along the lines of man /usr/local/src/regex/regex.7 in order to read it.


No external libraries are needed to build this extension.



Do not change the TYPE unless you know what you are doing.

To enable regexp support configure PHP --with-regex[=TYPE]. TYPE can be one of system, apache, php. The default is to use php.

The windows version of PHP has built in support for this extension. You do not need to load any additional extension in order to use these functions.

Runtime Configuration

This extension has no configuration directives defined in php.ini.

Resource Types

This extension has no resource types defined.

Predefined Constants

This extension has no constants defined.


Example 1. Regular Expression Examples

// Returns true if "abc" is found anywhere in $string.
ereg ("abc", $string);            

// Returns true if "abc" is found at the beginning of $string.
ereg ("^abc", $string);

// Returns true if "abc" is found at the end of $string.
ereg ("abc$", $string);

// Returns true if client browser is Netscape 2, 3 or MSIE 3.
eregi ("(ozilla.[23]|MSIE.3)", $HTTP_USER_AGENT);  

// Places three space separated words into $regs[1], $regs[2] and $regs[3].
ereg ("([[:alnum:]]+) ([[:alnum:]]+) ([[:alnum:]]+)", $string,$regs); 

// Put a <br /> tag at the beginning of $string.
$string = ereg_replace ("^", "<br />", $string); 
// Put a <br /> tag at the end of $string.
$string = ereg_replace ("$", "<br />", $string); 

// Get rid of any newline characters in $string.
$string = ereg_replace ("\n", "", $string);

See Also

For regular expressions in Perl-compatible syntax have a look at the PCRE functions. The simpler shell style wildcard pattern matching is provided by fnmatch().

Table of Contents
ereg_replace -- Replace regular expression
ereg -- Regular expression match
eregi_replace -- replace regular expression case insensitive
eregi -- case insensitive regular expression match
split -- split string into array by regular expression
spliti --  Split string into array by regular expression case insensitive
sql_regcase --  Make regular expression for case insensitive match

add a note add a note User Contributed Notes
Regular Expression Functions (POSIX Extended)
mina86 at tlen dot pl
20-Oct-2003 12:14
I tested how fast POSIX and Perl regular expresions are, and here are the results:

           | POSIX Extended  | Perl-Compatible |   POSIX - Perl
     match |    0.1296420097 |    0.1006720066 |  0.0289700031
   match i |    0.1204010248 |    0.1101620197 |  0.0102390051
   replace |    0.1896649599 |    0.1298999786 |  0.0597649813
 replace i |   10.6998120546 |    0.1453789473 | 10.5544331074

So, as you can see, preg_* functions are faster then ereg* functions. You can find source code of my test script here:
russlndr at online dot no
02-Jul-2003 08:55
The Regex Coach - interactive regular expressions:
tino at infeon dot com
12-Jun-2003 04:49
The book "Mastering Regular Expressions" is an invaluable resource.
Anand Thakur
26-Mar-2003 03:43
I saw a link to this page somewhere.  It is a library of user-submitted regular expressions for various things.  Some good stuff there.
16-Jan-2003 02:53
Ever wondered how to exclude "[" and "]"?
Here it goes: "[^][]". Extra characters to exclude can beadded right in the middle like this: "[^]fobar[]".
moc DOT liamtoh AT ssengnorw
18-Oct-2002 11:28
In a PCRE \s matches whitespace, but not inside a character class:

preg_match ('/\s/', ' ') // match
preg_match ('/[\s]/', ' ') // no match

Within a character class [:space:] is treated as a single character that matches any single whitespace character:

$pattern = '/[[:space:]]/';
$subject = "space tab\tnewline\n";
preg_match_all($pattern, $subject, $out) // == 3

To match a hyphen from within a character class, it must either be first or last; otherwise, it will act as a range operator.

Example: To match a blank string or a string containing only uppercase letters, underscores, spaces, and hyphens:

preg_match('/^[A-Z_ -]*$/', $subject)

To match any whitespace, not just spaces:

preg_match('/^[A-Z_[:space:]-]*$/', $subject)
09-Sep-2002 01:57
I have also experienced the same problem as had been experiencing, except I did not recognize the problem until after many hours of debugging.

"\s" does not seem to represent spaces, however "[[:space:]]" does.

Another problem I was having was matching dashes/hyphens '-'. You must escape them "\-" and place them at the end of a bracket expression.

Example: To match a blank string or a string containing only uppercase letters, underscores, spaces, and hyphens:


Hope this saves someone some time from debugging like I was. :)
bps7j at yahoo dot com
22-Aug-2002 09:40
Something that really got me: I'm used to using Perl's regexps, and so I used \s to check for a whitespace character in a password on a website. My PHP book (Wrox Press, Professional PHP Programming) agreed with me that this is exactly the same as [ \r\n\t\f\v], but it's NOT. In fact, what it did was keep anyone from joining the site if they put an 's' in their password! So beware, check for subtle differences between what you're used to and PHP.

[[:space:]] works fine, by the way.

I'm going to use the pcre functions from now on... I like Perl :o)
david at NOgreenhammerSPAM dot com
10-Mar-2002 02:40
Sadly, the Posix regexp evaluator (PHP 4.1.2) does not seem to support multi-character coallating sequences, even though such sequences are included in the man-page documentation.

Specifically, the man-page discusses the expression "[[.ch.]]*c" which matches the first five characters of "chchcc".  Running this expression in ereg_replace generates the error "Warning: REG_ECOLLATE".  (Running an equivalent expression with only one character between the periods does work, however.)

Multi-character coallating sequences are not supported!

This is really, really too bad, because it would have provided a simple way to exlude words from the target.

I'm going to go learn PCRE, now.  :-(
regex at dan42 dot cjb dot net
08-Mar-2002 02:33
Follow-up to my previous post:
Some simple optimization allowed me to realize that excluding a word at the beginning of a string has a degree of complexity O(n) rather than O(n^2). I only had to follow the logic:

if str[0] != badword[0] then OK
  if str[1] != badword[1] then OK
    if str[2] != badword[2] then OK
    else ...

So excluding the word 'abc' at the beginning of a string is much more simple than I had made it out to be:
spiceee at potentialvalleys dot com
07-Mar-2002 02:26
sorry to be picky here but saying ^ is beginning of a line or $ is end of line is rather misleading, if you're working on a daily basis with regexes.

it might be that it is most of the time correct BUT in some occasions you'd be better off to think of ^ as "start of string" and $ as "end of string".

there are ways to make your regex engine forget about your system's notion of a newline, it's what is commonly refered to as multiline regexes...
04-Mar-2002 04:15
mholdgate wrote a very nice quick reference guide in the next page (, but I felt it could be improved a little:

^        Start of line
$        End of line
n?        Zero or only one single occurrence of character 'n'
n*        Zero or more occurrences of character 'n'
n+        At least one or more occurrences of character 'n'
n{2}        Exactly two occurrences of 'n'
n{2,}        At least 2 or more occurrences of 'n'
n{2,4}        From 2 to 4 occurrences of 'n'
.        Any single character
()        Parenthesis to group expressions
(.*)        Zero or more occurrences of any single character, ie, anything!
(n|a)        Either 'n' or 'a'
[1-6]        Any single digit in the range between 1 and 6
[c-h]        Any single lower case letter in the range between c and h
[D-M]        Any single upper case letter in the range between D and M
[^a-z]        Any single character EXCEPT any lower case letter between a and z.

        Pitfall: the ^ symbol only acts as an EXCEPT rule if it is the
        very first character inside a range, and it denies the
        entire range including the ^ symbol itself if it appears again
        later in the range. Also remember that if it is the first
        character in the entire expression, it means "start of line".
        In any other place, it is always treated as a regular ^ symbol.
        In other words, you cannot deny a word with ^undesired_word
        or a group with ^(undesired_phrase).
        Read more detailed regex documentation to find out what is
        necessary to achieve this.

[_4^a-zA-Z]    Any single character which can be the underscore or the
        number 4 or the ^ symbol or any letter, lower or upper case

?, +, * and the {} count parameters can be appended not only to a single character, but also to a group() or a range[].

would mean:

^.{2}         = A line beginning with any two characters,
[a-z]{1,2}     = followed by either 1 or 2 lower case letters,
_?         = followed by an optional underscore,
[0-9]*         = followed by zero or more digits,
([1-6]|[a-f])     = followed by either a digit between 1 and 6 OR a
        lower case letter between a and f,
[^1-9]{2}     = followed by any two characters except digits
        between 1 and 9 (0 is possible),
a+$         = followed by at least one or more
        occurrences of 'a' at the end of a line.
regex at dan42 dot cjb dot net
21-Feb-2002 01:12
It's easy to exclude characters but excluding words with a regular expression is a bit more tricky. For parentheses there is no equivalent to the ^ for brackets. The only way I've found to exclude a string is to proceed by inverse logic: accept all the words that do NOT correspond to the string. So if you want to accept all strings except those _begining_ with "abc", you'd have to accept any string that matches one of the following:

which, put together, gives the regex

Note that this won't work to detect the word "abc" anywhere in a string. You need to have some way of anchoring the inverse word match
like: ^(a[^b]|[^a]b|[^a][^b])   ;"ab" not at begining of line
  or: (a[^b]|[^a]b|[^a][^b])&   ;"ab" not at end of line
  or: 123(a[^b]|[^a]b|[^a][^b]) ;"ab" not after "123"

I don't know why "(abc){0,0}" is an invalid synthax. It would've made all this much simpler.
Slightly off-topic, here's a regex date validator (format yyyy-mm-dd, remove all spaces and linefeeds):
03-Feb-2002 11:02
if you are looking for the abbreviations like tab, carriage return, regex-class definitions 

you should look here:

some excerpts:

    \a    control characters bell
    \b    backspace
    \f    form feed
    \n    line feed
    \r    carriage return
    \t    horizontal tab
    \v    vertical tab

class example
    \cLu    all uppercase letters
webmaster at datamike dot org
18-Dec-2001 09:39
I noticed Cyro's link had gone old. So I made copy of the regex manpage and placed it on my site. You can get it from the following address:

This is primarily for Windows users, who have no access to the man pages in Linux distributions.
bart at framers dot nl
07-Mar-2001 10:53
Dario seems to have made a nice tutorial about regular expressions:

Thanks Dario! ...
07-Mar-2001 03:38
If you don't have commandline access to the manpage cited above, note that the "POSIX 1003.2 Regular Expressions" manpage is also widely re-published on the web.  See, for instance:

The "POSIX 1003.2 Regular Expressions" manpage provides a good basic reference for the syntax used by ereg_* functions.  Most tutorials on "extended regular expressions" are also applicable.

 Last updated: Thu, 21 Aug 2003
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