Learning Regular Expressions

Meta Characters

  • Quotation marks are not meta characters

WildCard

/h.t/ matches “hat” “hot” and “hit” but not “heat”

Spaces

  • tabs: \t
  • line return: \r, \n, \r\n

Character Set

/gr[ae]y/ matches “grey” and “gray” /gr[ea]t/ does not match “great”

Ranges

Includes al characters between two characters [0-9] [A-Za-z] [50-99] is not all numbers from 50 to 99, it is the same as [0-9]

Negative Character set

/see[^mn]/ matches “seek” and “sees” but not “seem” or “seen”, does not match “see”

Meta Characters Inside Character Sets

Most meta characters inside character sets are already escaped /h[a.]t/ matches “hat” and “h.t”, but not “hot” Exceptions: ] - ^ \

Shorthand Character Sets

\d - digit [0-9] \w - word [a-zA-Z0-9_] \s - whitespace [\t\r\n]

\D - not digit [^0-9] \W - not word [^a-zA-Z0-9_] \S - not whitespace [^\t\r\n]

Repetition

    • zero or more times
    • one or more times ? - zero or one time

/.+/ matches any string of characters except a line return

/apples*/ matches “apple”, “apples”, and “applessssssss” /apples+/ matches “apples”, “applessssss”, but not “apple” /apples?/ matches “apple”, “apples”, but not “applesssssssss”

/\d\d\d\d*/ matches numbers with three digits or more /\d\d\d+/ matches numbers with three digits or more

/colou?r/ matches “color” and “colour”

Quatified Repetition

{min,max}

  • min and max are positive numbers
  • min must always be included and can be zero

\d{4,8} matches numbers with four to eight digits \d{4} matches number swith exactly four digits (min is max) \d{4,} matches numbers with four or more digits (max is infinite)

\d{0,} is the same as \d* \d{1,} is the same as \d+

/\d{3}-\d{3}-\d{4}/ matches most US phone numbers

/A{1,2} bonds/ matches “A bonds” and “AA bonds”, not “AAA bonds”

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 greed, but “gives back” the “.jpg”to make the match

Gives back as little as possible:

  • /.*[0-9]+/ matches “Page 266”
  • .* portion matches “
  • [0-9]+ portion matches only “6”
  • Match as much as possible before giving control to the next expression part

Regular expression engines are eager and greedy

Lazy Expressions

*? +? {min,max}? ??

Instructs quantifier to use a “lazy strategy” for making choices

  • Match as little as possible before control to the next expression part
  • Still defers to overall match
  • Mot necessarily faster or slower

/.*?[0-9]+/ trying to match “Page 266”, they try to match with wildcard as many times as a digit appears

Grouping and Alternating

/(abc)+/ matches “abc” and “abcabcabc” /(in)?dependent/ matches “independent” and “dependent” /run(s)?/ is the same as /runs?/

  • Group portions of the expression
  • Apply repetition operators to a group
  • Create a group of alternation expressions
  • Captures group for use in matching and replacing

Alternation Metacharacter

  • is an OR operator
  • Either match expression on the left or match expression on the right
  • Ordered, leftmost expression gets precedence
  • Multiple choices can be daisy-chained

  • Group alternation expressions to keep them distinct

    /w(ei|ie)rd/ matches “weird” and “wierd”

Archors

^ 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$/ or /apple\Z/
  • /^apple$/ or /\Aapple\Z/

Use “m” flag to match end and start on all lines

Work Boundaries

  • Reference a position, not an actual character
  • Before the first word character in the string
  • After the last word character in the string
  • Between a word character and a non-word character
  • workd characters [A-Za-z0-9_]

/\ba\b/ match all “a” that are alone