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Regular Expressions

syntax: /RegEx/flags

  • ( // ) - RegEx literal

  • RegEx - search pattern ( literal characters, meta characters )

  • Literal Characters - all alphanumeric characters.

  • Metacharacters - ( . ? ! ^ $ * + - = \ | : {} [] () )

  • Flags - g, i, m, u, y, s

    • g - global
    • i - case insensitive
    • m - multiline
    • s - single line (default)
    • u - ungreedy
    • y - extended

Characters

Wildcard Metacharacter ( . )

/./ - matches any character except line brakes.

Text: "9.00, 9500, 9-00, 9110"

/9.00/g

Match: 9.00 9500 9-00

Escaping Metacharacter ( \ )

  • Only escape metacharacter

  • /\./ - literal period not wildcard metacharacter "."

  • /\\/g - backslash literal character

Text: "9.00 9500 9-00"

/9\.00/g

Match: 9.00

Text: "/etc/"

/\/etc\//

Match: /etc/

Example

Text: "resume1.txt, resume2.txt, resume3_txt.zip"

/resume.\.txt/g

Match: resume1.txt resume2.txt

|| Space || Tab (\t) || Line return (\r, \n, \r\n) ||

/" "/

Match: " "

/.\t.\t./

Match: 1<tab>2<tab>3

/c\nd/
  • \r (Carriage Return), \n (Line Feed)

Match: c<CRLF>d

Character set

  • Character set metacharacter ( [] )

  • /[aeiou]/ - matches any one vowel "a", "e", "i", "o", "u"

  • /gr[ea]y/ - matches grey gray, but not "greay"

Character Ranges

  • Character Ranges metacharacter ( - )

  • Character Range metacharacter outside a character set ( [] ) act as a literal character

  • [0-9] - 0 to 9, [a-z] - a to z, [A-Z] - A to Z

  • [60-99] - 6, [0-9], 9 (i.e. from 0 to 9)

Text: 555-666-7890

/[0-9][0-9][0-9]-[0-9][0-9][0-9]-[0-9][0-9][0-9][0-9]/g

Match: 555-666-7890

Negative Character Set

  • Metacharacter ( ^ ) - Negates a entire character set

  • Not any one of several characters

  • Add ^ as the first character inside a character set

  • /^aeiou/g matches any one consonant (non-vowel)

  • /see[^mn]/g matches "seek", "see " and "sees" but not "seem", "seen" or "see"

Text: "It seems I see the sea I seek"

/see[^mn]/g

Match: "see " "seek"

Metacharacters inside a Character sets

  • Most metacharacters inside a character set are already escaped

  • /h[a.]t/ matches "hat", "h.t" but not "hot"

  • 13[-/]03[-/]2020 matches "13-03-2020, 13/03/2020/"

  • Exceptions : ( ] ^ - \ )

Text: "index[0], index(3), index(8]"

/index[[(][0-9][\])]/g

Match: "index[0]" "index(3)" "index(8]"

Text: "file01 file-1 file\1 file_1"

/file[0_\-\\][0-9]/g

Match: "file01" "file-1" "file\1" "file_1"

Shorthand Character Sets

Shorthand Meaning Equivalent
\d Digit [0-9]
\w Word Character [a-zA-Z0-9_]
\s Whitespace [\t\n\r ]
\D Not Digit [^0-9]
\W Not Word Character [^a-zA-Z0-9_]
\S Not Whitespace [^\t\n\r ]

/\d\d\d\d/ matches "1999", but not "text"

/\w\w\w/ matches "ABC", "aBc", "12a", and "a__", but not "a-a"

/\w\s\w\w/ matches "I am", but not "Am I"

/\D/ same as both /[^0-9]/ /[^\d]/

Caution

  • \w - [a-zA-Z0-9_]

    • Underscore is a word character

    • Hyphen is not a word character

  • /[^\d\s]/ is not same as /[\D\S]/

    • /[^\d\s]/ = Not Digit or Space Character

    • /[\D\S]/ = Not Digit or Not Space Character

/[\w\-]/ matches any word character or hyphen (useful)

Repetition Metacharacter

Metacharacter Meaning
* Preceding item, zero or more times
+ Preceding item, one or more times
? Preceding item, zero or one time

/.+/ - Matches any string of characters except a line return

Text: "Good Morning. Good Day. Good Evening. Good Night_"

/Good .+_/g

Match: "Good Morning. Good Day. Good Evening. Good Night_"

Text: "apple apples applessss"

/apples?/g

Match: "apple" "apples" "apples"

/apples*/g

Match: "apple" "apples" "applessss"

/apples+/g

Match: "apples" "applessss"

Quantified Repetition

Metacharacter Meaning
{ Start quantified repetition of preceding item
} End quantified repetition of preceding item

Three Syntaxes

  1. {min, max}

    • /\d{4, 8}/ - matches numbers with four to eight characters

  2. {min}

    • /\d{4}/ - matches numbers with exactly four digits (min is max)

  3. {min,}

    • /\d{4,}/ - matches numbers with four or more digits (infinite)

  • /\d{0,}/ - is same as /\d*/

  • /\d{1,}/ - is same as /\d+/

  • /\d{3}-\d{3}-\d{4}/ - matches 123-456-7890

  • /A{1,2} bonds/ - matches A bonds, and AA bonds

Text: "1. a 2. ab 3. abc 4. abcd 5. abcde 6. abcdef"

/\d. \w{3,5}/g

Match: "3. abc " "4. abcd " "5. abcde "

Greedy Expressions

  • Standard repetition quantifiers are greedy

  • Expression tries to match the longest possible string

  • Defers to achieving overall match

  • /.+\.jpg/ matches "filename.jpg"

  • The + is greedy, but "gives back" the .jpg to make the match

  • Gives back as little as possible

  • /.*[0-9]+/ matches "Page 266"

Text: "asdf", "asdf", "asdf, asdf."

/".+", ".+"/g
  1. ".+" - "asdf", "asdf",
  2. ".+" - "asdf, asdf."

Match: "asdf", "asdf", "asdf, asdf."

Lazy Expression

Metacharacter Meaning
? Make preceding quantifier lazy

  • Lazy Expression ( *?, +?, {min, max}?, ?? )

  • Instructs quantifier to use a "lazy strategy" for making choices

  • Match as little as possible before giving control to the next expression part

Text: "asdf", "asdf", "asdf, asdf."

/".+?", ".+?"/g

Match: "asdf", "asdf"

Grouping Metacharacters

Metacharacter Meaning
( Start Group Expression
) End Group Expression

  • A list of three use cases for grouping metacharacters.

    1. Repetition Operators
    2. Alternation
    3. Capturing for replacing

  • /(abc)+/ matches abc and abcabcabc

  • /(in)?dependent/ matches independent and dependent

  • /run(s)?/ is same as /runs?/

Capturing for replacing

  • (\d{3})-(\d{3}-\d{3}) matches 555-666-7890

    • (\d{3}) - $1
    • (\d{3}-\d{3}) - $2
    • ($1) $2 - "(555) 666-7890"

  • Non capturing group (?:) - (?:.+)

Alternation Metacharacter ( | - OR operator)

  • ( | ) matches previous or next expression
  • Ordered, leftmost expression gets precedence
  • /apple|orange/g matches apple and orange
  • /w(ie|ei)rd/g matches wierd and weird
  • /abc|def|ghi/g matches abc, def and ghi
  • /apple(juice|sauce)/g is not the same as /applejuice|sauce/g
  • /(AA|BB|CC){4}/g matches AABBCCAA CCBBCCBB AABBCCCC

Text: AABB AACC BBCC ACCBBA CCCC

/(AA|BB|CC){2}/g

Match: AABB AACC BBCC CCBB CCCC

Text: does not does no do not do no do nothing does nothing

/[Dd]o(es)? (nothing|not|no)/

Match: "does not" "does no" "do not" "do no" "do nothing" "does nothing"

  • Order (nothing|not|no) matters
  • (no|not|nothing) matches only no

Anchors

Start and End Anchors

Metacharacter Meaning
^ Start of string/line
$ End of string/line
\A Start of string, never end of line
\Z End of string, never end of line

  • Reference a position, not an actual character

  • Zero Width

  • /^apple/ or /\Aapple/ (apple must be the first word)

  • \A and \Z are not yet supported by JavaScript

  • /^apple/ matches apple only if it's beginning of a line (multiline flag)

Text: apple to apples to appless

/apple(ss|s)$/

Match: appless

Text: apple to apples to appless

/^apple(ss|s)?/

Match: apple

Line Breaks and Multiline mode

Text:

Apples and pears.
oranges
bananas
mangoes

/^[\w ,.]+$/g

Match:

Apples and pears.

Text:

Apples and pears.
oranges
bananas
mangoes

/^[\w ,.]+$/gm

Match:

Apples and pears.
oranges
bananas
mangoes

Word Boundaries

Metacharacter Meaning
\b Word Boundary (start/end of word)
\B Not a Word Boundary
  • Reference a position, not an actual character

String: This is a test

  • Before the first word character in the string ( \bThis )

  • After the last word character in the string ( test\b )

  • Word character [ a-zA-Z0-9_ ]

  • Between a Word character and a Non Word character ( This\b \bis\b \ba\b \btest )

  • /\b\w+\b matches four words in text "This is a test."

    • \bThis\b

    • \bis\b

    • \ba\b

    • \btest\b

  • /\bNew\bYork\b/ does not match New York

  • /\bNew\b \bYork\b/ matches New York

  • /\B\w+\B/ finds two matches in text "This is a test."

    • hi
    • es

  • /\b\w+s\b/ matches apples in "We picked apples."

Look Arounds

Capturing Group

  • Capturing Group - (...) ( $1 ( \1 ) )
  • Non Capturing Group - (?:...)
  • Named Group - (?<firstGroup>...) ( $firstGroup ( \firstGroup ) )
  • If conditionals - (($1)|($2)) (?(2)...| else...)
  • Look Arounds
    • Positive lookahead: (?=), Negative lookahead: (?!)
    • Positive lookbehind: (?<=), Negative lookbehind: (?<!)

Lookaheads

  • Positive lookahead: (?=), Negative lookahead: (?!)

  • Match an expression in a group but do not capture it; works as a boundary

  • Only Static lengths are supported in lookahead groups

Text: "53222 22222-1234 23a23323 12345-1234 33333-1234"

/\d{5}(?=-\d{4})/g

Match: 2222 12345 33333

Text: "53222 22222-1234 23a23323 12345-1234"

/\d{5}(?!-\d{4})/g

Match: 53222 23323

Lookbehinds

  • Positive lookbehind: (?<=), Negative lookbehind: (?<!)

  • Match an expression in a group but do not capture it; works as a boundary

  • Only Static lengths are supported in lookbehind groups

Text:

<li><code>$1</code></li>
<li><code>$2</code></li>
<code>$3</code>
<code>$4</code>

/(?<=<li><code>).+?(?=<\/code>)/g

Match: $1 $2

<li><code>$1</code></li>
<li><code>$2</code></li>
<code>$3</code>
<code>$4</code>

/(?<!<li><code>)(?<=<code>).+?(?=<\/code>)/g

Match: $3 $4

If conditionals

  • Named and Nested Conditionals

Text:

----------
          ID: mod_gd
    Function: pkg.installed
        Name: php-gd
      Result: True
     Comment: All specified packages are already installed
     Started: 16:19:04.848951
    Duration: 14.303 ms
     Changes:
----------
          ID: mod_mbstring
    Function: pkg.installed
        Name: php-mbstring
      Result: True
     Comment: All specified packages are already installed
     Started: 16:19:04.863402
    Duration: 13.866 ms
     Changes:
----------

/(ID|(?<fn>Function)|(?<dur>Duration)): (?(fn)\w+?\.\w+|(?(dur)\d+\.\d+ ms|\w+))/g

Match:

ID: mod_gd

Function: pkg.installed

Duration: 14.303 ms

ID: mod_mbstring

Function: pkg.installed

Duration: 13.866 ms