Regular expressions have many uses in any code which works with text strings (or streaming textual data). Regex find use across a wide variety of complex use cases, some examples:
colou?rfinds the similarly-spelled words color (American) and colour (Commonwealth)
[^ ]*cat[^ ,!]*finds the words containing the root "cat" in "A cat, not a dog, and the toys the cat’s scattered about — categorically a catastrophe!"
Some people, when confronted with a problem, think “I know, I’ll use regular expressions.” Now they have two problems.
A regex has the syntax
/pattern/modifiers. Using the string
/cat/i as an example, regex terminology is:
/cat/iis a complete regular expression
catis the pattern
iis the modifier
- the forward-slashes / are the pattern delimiters
search() function takes a regular expression to search for a match, and returns the position of the match.
replace() function takes a regular expression and returns a modified string where the pattern is replaced.
search() function takes a regex and returns the position within the string where the match is found (or a -1 if not found). For example, here we search through
string for "Cat" with the
imodifier (ignore case during the search).
let regex = /Cat/i ; // case-insensitive search for "Cat" let string = "my cat, Cat" ; // search in this string console.log( string.search( regex ) ) ; // --> 3
Equivalent to the above,
search() may be invoked directly on any string variable:
console.log( "my cat, Cat".search(/Cat/) ) ; // --> 10 console.log( "my cat, Cat".search(/Cat/i) ) ; // --> 3
/Cat/ is a very different beast than is
"Cat" in this context: the former is a regex, the latter a string. It is a frequent source of debugging rage to think one is searching with the power of regex matching only to discover a quoted string. Read on for mention of the
RegExp() function, which makes this kind of typo less common.
replace() function takes a regex and returns a string, possibly modified by the pattern if the search is successful. Compare the results of the following three calls:
Search by a quoted literal for the first occurrence:
console.log( "my cat, Cat".replace("Cat", "Dog") ) ; // --> my cat, Dog
Search by a regex, insensitively, for the first occurrence:
console.log( "my cat, Cat".replace(/Cat/i, "Dog") ) ; // --> my Dog, Cat
Search by a regex, insensitively, globally (all occurrences):
console.log( "my cat, Cat".replace(/Cat/ig, "Dog") ) ; // --> my Dog, Dog
Changing Regular Expression Evaluation
The behavior of the regular expression matching engine is changed and extended through the use of modifiers, character ranges, class type meta-characters, and quantifiers.
Modifiers change the default matching behavior (return the first match, use case-sensitive matching, and match only the first line of a multi-line variable):
|g||global matching (all, rather first only)|
Ranges, delimited by square brackets
, match a range of characters:
||Find any character between the brackets|
||Find any character NOT between the brackets|
||Find any character between the brackets (any digit)|
||Find any character NOT between the brackets (any non-digit)|
Meta-characters match specific kinds of characters:
||a single character, except newline or line terminator|
||a word character|
||a non-word character|
||a non-digit character|
||a whitespace character|
||a non-whitespace character|
||a match at a word boundary, at the beginning with
||a match, but not at the beginning or end of a word|
||new line character|
||form feed character|
||carriage return character|
||vertical tab character|
||the character specified by an octal number xxx|
||the character specified by a hexadecimal number
||the Unicode character specified by a hexadecimal number
Quantifiers change the specific number match sequences:
||Match at least one
||Match zero or more occurrences of
||Match zero or one occurrences of
||Match a sequence of X
||Match a sequence of X to Y
||Match a sequence of at least X
||Match any string followed by a specific string
||Match any string not followed by a specific string
RegExp object provides a more flexible mechanism for using variables for each of the matching elements. You’ve been watching it in action — the forward slashes automagically create regexps — so the following are equivalent:
let foo = new RegExp("is a") ; let bar = /is a/ ;
test() and exec()
Let’s begin exploring the
RegExp object with running a literal string match on a literal string object:
console.log( /Cat/ig.exec( "My cat, Thor, is a Bombay." )) ;
exec() methods to return a boolean and a results array, respectively:
let string = "My cat, Thor, is a Bombay cat." ; let pattern = /Cat/ig ; console.log( pattern.test( string )) ; // --> true console.log( pattern.exec( string )) ; // --> ["cat"]
search() method returns the location of the first match:
console.log( string.search( pattern )) ; // --> 3
split() method treats the pattern as a delimiter and returns an array with all the string cut apart by the pattern.
let string = "My cat, Thor, is a Bombay cat." ; let pattern = /Cat/ig ; console.log( string.split( pattern )) ; // --> ["My ", ", Thor, is a Bombay ", "."]
match() method returns an array with all the string matches.
console.log( string.match( pattern )) ; // --> ["cat","cat"]
That’s not very impressive, but consider the search results (from the example given far above) — now some of the power of
match() becomes evident:
let string = "A cat, not a dog, and the toys the cat's scattered about — categorically a catastrophe!" ; let pattern = /[^ ]*cat[^ ,!]*/g ; console.log( string.match( pattern )) ; // --> [ "cat", "cat's", "scattered", "categorically", "catastrophe" ]
let string = "A cat, not a dog, and the toys the cat's scattered about — categorically a catastrophe!" ; let pattern = /Cat/ig ; let replacement = "Dog" ; console.log( string.replace( pattern, replacement )) ;
The output shows that we might need more work to get satisfactory results:
A Dog, not a dog, and the toys the Dog’s sDogtered about — Dogegorically a Dogastrophe!
As powerful as regex are, mention must be made of grouping, which provides a short-term memory of the matches found.
In the following code snippet, we’re matching two words
\w+ that are separated by any whitespace character
\s (which may be a space, tab, carriage return, newline, vertical tab, or form feed character).
The matches, remembered by the grouping parenthesis, are recalled in the replacement specification by the positional markers
$2 and reversed by changing their order in the string
let regex = /(\w+)\s(\w+)/ let string = 'Firstname Lastname' console.log( string.replace(regex, '$2, $1')) ; // --> Lastname, Firstname
- the syntax of modifiers, ranges, meta-characters, and quantifiers
- regular expression groups and positional variables