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Regular Expressions I

Regular Expressions are a concise method of describing text patterns. In JavaScript a Regular Expression are also special objects that can be used with some methods to help find or transform text.

Defining a Regular Expression:

  1. let r = /ab+c/ constructing with slashes.
  2. let r = new RegExp('ab+c') constructing with the constructor function 'RegExp'.

Special Characters

Regular Expression use a bunch of special characters to express pattern matches.

Character meaning example
^ Matches beginning of input
$ Matches end of input
* Matches preceding expression 0 or more times
+ Matches preceding expression 1 or more times
? Matches preceding expression 0 or 1 times
. Matches any single character except newline
{n} Matches exactly 'n' occurrences of the preceding expression /b{3}/
n, Matches at least 'n' occurrences of the preceding expression /b{3,}/
n,m Matches at least 'n' and at most 'm' occurrences of the preceding expression /b{3,}/
[abc] Matches any one of 'a' or 'b' or 'c'
abc Matches anything not 'a' or 'b' or 'c'
\b Matches a word boundary
\B Matches a non-word boundary
\d Matches a digit character same as [0-9]
\D Matches a non-digit character same as [^0-9]
\s Matches a white space character
\S Matches a non-white space character
\w Matches any alphanumeric character same as [A-Za-z0-9_]
\W Matches any alphanumeric character same as [^A-Za-z0-9_]
| or ie. matches 'a' or 'b' a | b
(x) matches 'x' and remembers the match
(?:x) matches 'x' but does not rember the match

Regular expression flags

flags for regular expression change the search matching.

g - global search

i - case-insensitive

m - multi-line

u - unicode

y - sticky (starts with the current position in the target string)

Other sites about Regular Expressions