3.2 javascript regex

JavaScript: Regex
Regular expressions

2
What is a regular expression?
/[a-zA-Z_-]+@(([a-zA-Z_-])+.)+[a-zA-Z]{2,4}/
• regular expression ("regex"): describes a pattern of text
 can test whether a string matches the expr's pattern
 can use a regex to search/replace characters in a string
 very powerful, but tough to read
• regular expressions occur in many places:
 text editors (TextPad) allow regexes in search/replace
 languages: JavaScript; Java Scanner, String split
 Unix/Linux/Mac shell commands (grep, sed, find, etc.)

3
String regexp methods
.match(regexp) returns first match for this string
against the given regular
expression; if global /g flag is
used, returns array of all matches
.replace(regexp, text) replaces first occurrence of the
regular expression with the given
text; if global /g flag is used,
replaces all occurrences
.search(regexp) returns first index where the
given regular expression occurs
.split(delimiter[,limit]) breaks apart a string into an array
of strings using the given regular
as the delimiter; returns the
array of tokens

4
Basic regexes
/abc/
• a regular expression literal in JS is written /pattern/
• the simplest regexes simply match a given substring
• the above regex matches any line containing "abc"
 YES : "abc", "abcdef", "defabc", ".=.abc.=."
 NO : "fedcba", "ab c", "AbC", "Bash", ...

5
Wildcards and anchors
. (a dot) matches any character except n
 /.oo.y/ matches "Doocy", "goofy", "LooPy", ...
 use . to literally match a dot . character
^ matches the beginning of a line; $ the end
 /^if$/ matches lines that consist entirely of if
< demands that pattern is the beginning of a word;
> demands that pattern is the end of a word
 /<for>/ matches lines that contain the word "for"

7
String match
string.match(regex)
• if string fits pattern, returns matching text; else null
 can be used as a Boolean truthy/falsey test:
if (name.match(/[a-z]+/)) { ... }
• g after regex for array of global matches
 "obama".match(/.a/g) returns ["ba", "ma"]
• i after regex for case-insensitive match
 name.match(/Marty/i) matches "marty", "MaRtY"

8
String replace
string.replace(regex, "text")
• replaces first occurrence of pattern with the given text
 var state = "Mississippi";
state.replace(/s/, "x") returns "Mixsissippi"
• g after regex to replace all occurrences
 state.replace(/s/g, "x") returns "Mixxixxippi"
• returns the modified string as its result; must be stored
 state = state.replace(/s/g, "x");

10
Special characters
| means OR
 /abc|def|g/ matches lines with "abc", "def", or "g"
 precedence: ^Subject|Date: vs. ^(Subject|Date):
 There's no AND & symbol.
() are for grouping
 /(Homer|Marge) Simpson/ matches lines containing
"Homer Simpson" or "Marge Simpson"
starts an escape sequence
 many characters must be escaped: / $ . [ ] ( ) ^ * + ?
 ".n" matches lines containing ".n"

11
Quantifiers: * + ?
* means 0 or more occurrences
 /abc*/ matches "ab", "abc", "abcc", "abccc", ...
 /a(bc)/" matches "a", "abc", "abcbc", "abcbcbc", ...
 /a.*a/ matches "aa", "aba", "a8qa", "a!?_a", ...
+ means 1 or more occurrences
 /a(bc)+/ matches "abc", "abcbc", "abcbcbc", ...
 /Goo+gle/ matches "Google", "Gooogle", "Goooogle",
...
? means 0 or 1 occurrences
 /Martina?/ matches lines with "Martin" or "Martina"
 /Dan(iel)?/ matches lines with "Dan" or "Daniel"

12
More quantifiers
{min,max} means between min and max occurrences
 /a(bc){2,4}/ matches lines that contain
"abcbc", "abcbcbc", or "abcbcbcbc"
• min or max may be omitted to specify any number
 {2,} 2 or more
 {,6} up to 6
 {3} exactly 3

13
Character sets
[ ] group characters into a character set;
will match any single character from the set
 /[bcd]art/ matches lines with "bart", "cart", and "dart"
 equivalent to /(b|c|d)art/ but shorter
• inside [], most modifier keys act as normal characters
 /what[.!*?]*/ matches "what", "what.", "what!",
"what?**!", ...
– Exercise : Match letter grades e.g. A+, B-, D.

14
Character ranges
• inside a character set, specify a range of chars with -
 /[a-z]/ matches any lowercase letter
 /[a-zA-Z0-9]/ matches any letter or digit
• an initial ^ inside a character set negates it
 /[^abcd]/ matches any character but a, b, c, or d
• inside a character set, - must be escaped to be matched
 /[-+]?[0-9]+/ matches optional - or +, followed by at
least one digit
– Exercise : Match phone numbers, e.g. 206-685-2181 .

15
Built-in character ranges
• b word boundary (e.g. spaces between words)
• B non-word boundary
• d any digit; equivalent to [0-9]
• D any non-digit; equivalent to [^0-9]
• s any whitespace character; [ fnrtv...]
• s any non-whitespace character
• w any word character; [A-Za-z0-9_]
• W any non-word character
• xhh, uhhhh the given hex/Unicode character
 /w+s+w+/ matches two space-separated words

16
Regex flags
/pattern/g global; match/replace all occurrences
/pattern/i case-insensitive
/pattern/m multi-line mode
/pattern/y "sticky" search, starts from a given index
• flags can be combined:
/abc/gi matches all occurrences of abc, AbC, aBc, ABC, ...

17
Back-references
• text "captured" in () is given an internal number;
use number to refer to it elsewhere in the pattern
 0 is the overall pattern,
 1 is the first parenthetical capture, 2 the second, ...
 Example: "A" surrounded by same character: /(.)A1/
 variations
– (?:text) match text but don't capture
– a(?=b) capture pattern b but only if preceded by a
– a(?!b) capture pattern b but only if not preceded by a

18
Replacing with back-references
• you can use back-references when replacing text:
 refer to captures as $number in the replacement string
 Example: to swap a last name with a first name:
var name = "Durden, Tyler";
name = name.replace(/(w+),s+(w+)/, "$2 $1");
// "Tyler Durden"
– Exercise : Reformat phone numbers from 206-685-2181
format to (206) 685.2181 format.

19
The RegExp object
new RegExp(string)
new RegExp(string, flags)
• constructs a regex dynamically based on a given string
var r = /ab+c/gi; is equivalent to
var r = new RegExp("ab+c", "gi");
 useful when you don't know regex's pattern until runtime
– Example: Prompt user for his/her name, then search for it.
– Example: The empty regex (think about it).

20
Working with RegExp
• in a regex literal, forward slashes must be escaped:
/http[s]?://w+.com/
• in a new RegExp object, the pattern is a string, so the
usual escapes are necessary (quotes, backslashes, etc.):
new RegExp("http[s]?://w+.com")
• a RegExp object has various properties/methods:
 properties: global, ignoreCase, lastIndex,
multiline, source, sticky; methods: exec, test

21
Regexes in editors and tools
• Many editors allow regexes in their Find/Replace feature
• many command-line Linux/Mac tools support regexes
grep -e "[pP]hone.*206[0-9]{7}" contacts.txt

3.2 javascript regex

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3.2 javascript regex