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PHP -Regular Expressions

Regular expressions are a powerful tool for examining and modifying text. Regular expressions themselves, with a general pattern notation almost like a mini programming language, allow you to describe and parse text. They enable you to search for patterns within a string, extracting matches flexibly and precisely. However, you should note that because regular expressions are more powerful, they are also slower than the more basic string functions. You should only use regular expressions if you have a particular need.

examples:
Regular Expression Description
[abc] a, b, or c
[a-z] Any lowercase letter
[^A-Z] Any character that is not a uppercase letter
(gif|jpg) Matches either “gif” or “jpeg”
[a-z]+ One or more lowercase letters
[0-9.-] Аny number, dot, or minus sign
^[a-zA-Z0-9_]{1,}$ Any word of at least one letter, number or _
([wx])([yz]) wy, wz, xy, or xz
[^A-Za-z0-9] Any symbol (not a number or a letter)
([A-Z]{3}|[0-9]{4}) Matches three letters or four numbers
Perl-Compatible Regular Expressions emulate the Perl syntax for patterns, which means that each pattern must be enclosed in a pair of delimiters. Usually, the slash (/) character is used. For instance, /pattern/.

Brackets

Brackets ([]) have a special meaning when used in the context of regular expressions. They are used to find a range of characters.
Expression Description
[0-9] It matches any decimal digit from 0 through 9.
[a-z] It matches any character from lowercase a through lowercase z.
[A-Z] It matches any character from uppercase A through uppercase Z.
[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 pecial character having a specific connotation. The +, *, ?, {int. range}, and $ flags all follow a character sequence.
Expression Description
p+ It matches any string containing at least one p.
p* It matches any string containing zero or more p’s.
p? It matches any string containing zero or more p’s. This is just an alternative way to use p*.
p{N} It matches any string containing a sequence of N p’s
p{2,3} It matches any string containing a sequence of two or three p’s.
p{2, } It matches any string containing a sequence of at least two p’s.
p$ It matches any string with p at the end of it.
^p It matches any string with p at the beginning of it.

Examples:

Following examples will clear your concepts about matching chracters.
Expression Description
[^a-zA-Z] It matches any string not containing any of the characters ranging from a through z and A through Z.
p.p It matches any string containing p, followed by any character, in turn followed by another p.
^.{2}$ It matches any string containing exactly two characters.
<b>(.*)</b> It matches any string enclosed within <b> and </b>.
p(hp)* It matches any string containing a p followed by zero or more instances of the sequence hp.

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:
Expression Description
[[:alpha:]] It matches any string containing alphabetic characters aA through zZ.
[[:digit:]] It matches any string containing numerical digits 0 through 9.
[[:alnum:]] It matches any string containing alphanumeric characters aA through zZ and 0 through 9.
[[:space:]] It matches any string containing a space.

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