PHP
Regular Expressions
Regular expressions are nothing more than a
sequence or pattern of characters itself. They provide the foundation for
pattern-matching functionality.
Using regular expression you can search a
particular string inside a another string, you can
replace one string by another string and you can split a string into many
chunks.
PHP offers functions specific to two sets of
regular expression functions, each corresponding to a certain type of regular
expression. You can use any of them based on your comfort.
POSIX Regular
Expressions
The structure of a POSIX regular expression is not
dissimilar to that of a typical arithmetic expression: various elements
(operators) are combined to form more complex expressions.
The simplest regular expression is one that matches
a single character, such as g, inside strings such as g, haggle, or bag.
Lets give explanation for few concepts being used in
POSIX regular expression. After that we will introduce you with regular
expression related functions.
Brackets
Brackets ([]) have a special meaning when used in
the context of regular expressions. They are used to find a range of
characters.
Sr.No |
Expression & Description |
1 |
[0-9] It matches any decimal digit from 0 through 9. |
2 |
[a-z] It matches any character from lower-case a
through lowercase z. |
3 |
[A-Z] It matches any character from uppercase A through
uppercase Z. |
4 |
[a-Z] It matches any character from lowercase a through
uppercase Z. |
The ranges shown above are general; you could also
use the range [0-3] to match any decimal digit ranging from 0 through 3, or the
range [b-v] to match any lowercase character ranging from b through v.
Quantifiers
The frequency or position of bracketed character
sequences and single characters can be denoted by a special character. Each
special character having a specific connotation. The +, *, ?,
{int. range}, and $ flags all follow a character sequence.
Sr.No |
Expression & Description |
1 |
p+ It matches any string containing at least one p. |
2 |
p* It matches any string containing zero or more
p's. |
3 |
p? It matches any string containing zero or more
p's. This is just an alternative way to use p*. |
4 |
p{N} It matches any string containing a sequence
of N p's |
5 |
p{2,3} It matches any string containing a sequence of
two or three p's. |
6 |
p{2, } It matches any string containing a sequence of at
least two p's. |
7 |
p$ It matches any string with p at the end of it. |
8 |
^p It matches any string with p at the beginning of
it. |
Examples
Following examples will clear your concepts about
matching characters.
Sr.No |
Expression & Description |
1 |
[^a-zA-Z] It matches any string not containing any of the characters
ranging from a through z and A through Z. |
2 |
p.p It matches any string containing p, followed by
any character, in turn followed by another p. |
3 |
^.{2}$ It matches any string containing exactly two
characters. |
4 |
<b>(.*)</b> It matches any string enclosed within <b>
and </b>. |
5 |
p(hp)* It matches any string containing a p followed by
zero or more instances of the sequence php. |
Predefined Character Ranges
For your programming convenience several predefined
character ranges, also known as character classes, are available. Character
classes specify an entire range of characters, for example, the alphabet or an
integer set −
Sr.No |
Expression & Description |
1 |
[[:alpha:]] It matches any string containing alphabetic
characters aA through zZ. |
2 |
[[:digit:]] It matches any string containing numerical digits
0 through 9. |
3 |
[[:alnum:]] It matches any string containing alphanumeric
characters aA through zZ
and 0 through 9. |
4 |
[[:space:]] It matches any string containing a space. |
PHP's Regexp POSIX Functions
PHP currently offers seven functions for searching
strings using POSIX-style regular expressions −
Sr.No |
Function & Description |
1 |
ereg() The ereg() function
searches a string specified by string for a string specified by pattern,
returning true if the pattern is found, and false otherwise. |
2 |
ereg_replace() The ereg_replace() function searches for string specified by pattern and
replaces pattern with replacement if found. |
3 |
eregi() The eregi() function searches throughout a string specified by pattern
for a string specified by string. The search is not case sensitive. |
4 |
eregi_replace() The eregi_replace() function operates exactly like ereg_replace(),
except that the search for pattern in string is not case sensitive. |
5 |
split() The split() function
will divide a string into various elements, the boundaries of each element based
on the occurrence of pattern in string. |
6 |
spliti() The spliti() function operates exactly in the same manner as its
sibling split(), except that it is not case sensitive. |
7 |
sql_regcase() The sql_regcase() function can be thought of as a utility function,
converting each character in the input parameter string into a bracketed
expression containing two characters. |
PERL Style
Regular Expressions
Perl-style regular expressions are similar to their
POSIX counterparts. The POSIX syntax can be used almost interchangeably with
the Perl-style regular expression functions. In fact, you can use any of the quantifiers
introduced in the previous POSIX section.
Lets give explanation for few concepts being used in
PERL regular expressions. After that we will introduce you wih
regular expression related functions.
Meta characters
A meta character is simply
an alphabetical character preceded by a backslash that acts to give the
combination a special meaning.
For instance, you can search for large money sums
using the '\d' meta character: /([\d]+)000/,
Here \d will search for any string of numerical character.
Following is the list of meta
characters which can be used in PERL Style Regular Expressions.
Character Description
. a single
character
\s a whitespace character (space,
tab, newline)
\S non-whitespace character
\d a digit (0-9)
\D a non-digit
\w a word character (a-z, A-Z, 0-9,
_)
\W a non-word character
[aeiou] matches a single character in the given
set
[^aeiou] matches
a single character outside the given set
(foo|bar|baz) matches
any of the alternatives specified
Modifiers
Several modifiers are available that can make your
work with regexps much easier, like case sensitivity,
searching in multiple lines etc.
Modifier Description
i Makes
the match case insensitive
m Specifies
that if the string has newline or carriage
return
characters, the ^ and $ operators will now
match
against a newline boundary, instead of a
string
boundary
o Evaluates
the expression only once
s Allows
use of . to match a newline character
x Allows
you to use white space in the expression for clarity
g Globally
finds all matches
cg Allows
a search to continue even after a global match fails
PHP's Regexp PERL Compatible Functions
PHP offers following functions for searching
strings using Perl-compatible regular expressions −
Sr.No |
Function & Description |
1 |
preg_match() The preg_match() function searches string for pattern, returning true if
pattern exists, and false otherwise. |
2 |
preg_match_all() The preg_match_all() function matches all
occurrences of pattern in string. |
3 |
preg_replace() The preg_replace() function operates just like ereg_replace(),
except that regular expressions can be used in the pattern and replacement
input parameters. |
4 |
preg_split() The preg_split() function operates exactly like split(), except that
regular expressions are accepted as input parameters for pattern. |
5 |
preg_grep() The preg_grep() function searches all elements of input_array,
returning all elements matching the regexp pattern. |
6 |
preg_ quote() Quote regular expression characters |