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
- PERL Style Regular
Expressions
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 one p's.
|
|
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
|
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
|
The ereg_replace() function
searches for string specified by pattern and replaces pattern with
replacement if found.
|
|
3
|
The eregi() function searches
throughout a string specified by pattern for a string specified by string.
The search is not case sensitive.
|
|
4
|
The eregi_replace() function
operates exactly like ereg_replace(), except that the search for pattern in
string is not case sensitive.
|
|
5
|
The split() function will divide a
string into various elements, the boundaries of each element based on the
occurrence of pattern in string.
|
|
6
|
The spliti() function operates
exactly in the same manner as its sibling split(), except that it is not case
sensitive.
|
|
7
|
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
|
The preg_match() function searches
string for pattern, returning true if pattern exists, and false otherwise.
|
|
2
|
The preg_match_all() function
matches all occurrences of pattern in string.
|
|
3
|
The preg_replace() function
operates just like ereg_replace(), except that regular expressions can be
used in the pattern and replacement input parameters.
|
|
4
|
The preg_split() function operates
exactly like split(), except that regular expressions are accepted as input
parameters for pattern.
|
|
5
|
The preg_grep() function searches
all elements of input_array, returning all elements matching the regexp
pattern.
|
|
6
|
Quote regular expression characters
|
No comments:
Post a Comment
Give your valuable feedback