A simple and straight forward reference to regular expression
- Repeaters : * , + and { } :
These symbols act as repeaters and tell the computer that the preceding character is to be used for more than just one time. - The asterisk symbol ( * ):
It tells the computer to match the preceding character (or set of characters) for 0 or more times (upto infinite).Example : The regular expression ab*c will give ac, abc, abbc, abbbc….ans so on
- The Plus symbol ( + ):
It tells the computer to repeat the preceding character (or set of characters) for atleast one or more times(upto infinite).Example : The regular expression ab+c will give abc, abbc, abbc, … and so on.
- The curly braces {…}:
It tells the computer to repeat the preceding character (or set of characters) for as many times as the value inside this bracket.Example : {2} means that the preceding character is to be repeated 2 times, {min,} means the preceding character is matches min or more times. {min,max} means that the preceding character is repeated at least min & at most max times.
- Wildcard – ( . )
The dot symbol can take place of any other symbol, that is why it
is called the wildcard character.Example : The Regular expression .* will tell the computer that any character can be used any number of times.
- Optional character – ( ? )
This symbol tells the computer that the preceding character may
or may not be present in the string to be matched.Example : We may write the format for document file as – “docx?” The ‘?’ tells the computer that x may or may not be present in the name of file format.
- The caret ( ^ ) symbol: Setting position for match :tells the computer that the match must start at the beginning of the string or line.
Example : ^\d{3} will match with patterns like "901" in "901-333-".
- The dollar ( $ ) symbol
It tells the computer that the match must occur at the end of the string or before \n at the end of the line or string.Example : -\d{3}$ will match with patterns like "-333" in "-901-333".
- Character Classes
A character class matches any one of a set of characters. It is used to match the most basic element of a language like a letter, a digit, space, a symbol etc./s : matches any whitespace characters such as space and tab
/S : matches any non-whitespace characters
/d : matches any digit character
/D : matches any non-digit characters
/w : matches any word character (basically alpha-numeric)
/W : matches any non-word character
/b : matches any word boundary (this would include spaces, dashes, commas, semi-colons, etc)[set_of_characters] – Matches any single character in set_of_characters. By default, the match is case-sensitive.Example : [abc] will match characters a,b and c in any string.
[^set_of_characters] – Negation: Matches any single character that is not in set_of_characters. By default, the match is case sensitive.
Example : [^abc] will match any character except a,b,c .
[first-last] – Character range: Matches any single character in the range from first to last.
Example : [a-zA-z] will match any character from a to z or A to Z.
- The Escape Symbol : \ If you want to match for the actual ‘+’, ‘.’ etc characters, add a backslash( \ ) before that character. This will tell the computer to treat the following character as a search character and consider it for matching pattern.
Example : \d+[\+-x\*]\d+ will match patterns like "2+2" and "3*9" in "(2+2) * 3*9".
- Grouping Characters ( )A set of different symbols of a regular expression can be grouped together to act as a single unit and behave as a block, for this, you need to wrap the regular expression in the parenthesis( ).
Example : ([A-Z]\w+) contains two different elements of the regular expression combined together. This expression will match any pattern containing uppercase letter followed by any character.
- Vertical Bar ( | ) :
Matches any one element separated by the vertical bar (|) character.Example : th(e|is|at) will match words - the, this and that.
- \number :
Backreference: allows a previously matched sub-expression(expression captured or enclosed within circular brackets ) to be identified subsequently in the same regular expression. \n means that group enclosed within the n-th bracket will be repeated at current position.Example : ([a-z])\1 will match “ee” in Geek because the character at second position is same as character at position 1 of the match.
- Comment : (?# comment) –
Inline comment: The comment ends at the first closing parenthesis.Example : \bA(?#This is an inline comment)\w+\b
# [to end of line] : X-mode comment. The comment starts at an unescaped # and continues to the end of the line.
Example : (?x)\bA\w+\b#Matches words starting with A
I am the founder and developer at TECHXT. Over the decade I have helped hundreds build their own site. All plugins I develop are made have a common goal: Make it as simple as possible and as easy to use as possible.
Sudipto
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