Wednesday, September 30, 2020

PHP Regular Expressions



Hello folks! welcome back to another section of my tutorial on PHP. In this tutorial guide, we will be studying about the PHP Regular Expressions.

Regular expressions are nothing more than a pattern or series of characters itself. They provide the foundation for pattern-matching functionality.

Using regular expressions, you can search for a specific string inside another string, you can replace a string by another string and you can divide a string into so many chunks.

PHP offers functions specific to two sets of regular expression functions, with each conforming to a certain type of RegExp. You can use any of them based on your requirement.

  • POSIX Regular Expressions
  • PERL Style ReExp

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POSIX Regular Expressions

The structure of a POSIX regular expression is not different from that of an arithmetic expression; various elements (operators) are merged to form more complex expressions.

The simplest regular expression is the one that matches a single character, such as g, inside the strings, such as g, bottle, or bag.

Let us give some explanations for for few concepts that are being used in POSIX regular expression. After that, i am going to introduce you with regular expression related functions.

Brackets

The brackets ([]) have a special meaning when used in context of regular expressions. They are used in PHP program for finding a range of characters.

Sr.NoExpression & 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 above displayed ranges are general; you could also use the range [0-5] to match any decimal digit ranging from 0 through 5, or the range [b-v] for matching any lowercase character that ranges from b through v.

Quantifiers

The frequency or the position of bracketed character sequences and the 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.NoExpression & 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.



Example

Following example below will clear your concepts about the matching characters.

Sr.NoExpression & 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 coding benefits, several predefined character ranges, are available. The character ranges are used for specifying an entire range of characters, for example, the alphabet or an integer set -

Sr.NoExpression & 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 provides seven functions for searching strings using the POSIX-style regular expressions -

Sr.NoFunction & Description
1ereg()

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.

2ereg_replace()

The ereg_replace() function searches for string specified by pattern and replaces pattern with replacement if found.

3eregi()

The eregi() function searches throughout a string specified by pattern for a string specified by string. The search is not case sensitive.

4eregi_replace()

The eregi_replace() function operates exactly like ereg_replace(), except that the search for pattern in string is not case sensitive.

5split()

The split() function will divide a string into various elements, the boundaries of each element based on the occurrence of pattern in string.

6spliti()

The spliti() function operates exactly in the same manner as its sibling split(), except that it is not case sensitive.

7sql_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.


We are going to be discussing about the above listed functions in our subsequent tutorials.

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PERL Style RegExp

The perl-style regular expression are similar to their POSIX peer. You can make use of the POSIX syntax almost interchangeably with Perl-style regular expression functions. As a matter of fact, you can make use of any quantifiers found in the previous POSIX section.

Let us give an explanation for few concepts that are used in PERL regular expressions. After that we will introduce you with the regular expression related functions.

Meta Characters

A meta character is directly an alphabetical character preceded by a backslash that acts to give the combinations a very special meaning.

For instance, you can search for large money sums using the '\d' meta character: /([\d]+)000/. So here \d will search for any string of numeric character.

The following below is the list of meta characters that can be used in PERL Style Regexp.

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

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Modifiers
Many modifiers are available that can make your work with Regexp much simpler, 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 Functions
PHP offers the following function for searching strings using perl-compatible regular expressions -

Sr.NoFunction & Description
1preg_match()

The preg_match() function searches string for pattern, returning true if pattern exists, and false otherwise.

2preg_match_all()

The preg_match_all() function matches all occurrences of pattern in string.

3preg_replace()

The preg_replace() function operates just like ereg_replace(), except that regular expressions can be used in the pattern and replacement input parameters.

4preg_split()

The preg_split() function operates exactly like split(), except that regular expressions are accepted as input parameters for pattern.

5preg_grep()

The preg_grep() function searches all elements of input_array, returning all elements matching the regexp pattern.

6preg_ quote()

Quote regular expression characters


We are going to be discussing about the above listed functions in our subsequent tutorials.

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Alright guys! This is where we are rounding up for this tutorial post. In my next tutorial post, we will be discussing about the PHP ereg() Function.

Feel free to ask your questions where necessary and i will attend to them as soon as possible. If this tutorial was helpful to you, you can use the share button to share this tutorial.

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Thanks for reading and bye for now.
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