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regular_expression.md

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Regular Expression

Basic Concepts

Boolean "or"

A vertical bar(|) separates alternatives.

  • gray|grey # match "gray" or "grey"

Grouping

Parentheses are used to define the scope and precedence of the operators

  • gr(a|e)y # also match "gray" or "grey"

Quantification

A quantifier after a token (such as a character) or group specifies how often that preceding element is allowed to occur.

  • . Matches any single character (many applications exclude newlines, and exactly which characters are considered newlines is flavor-, character-encoding-, and platform-specific, but it is safe to assume that the line feed character is included). Within POSIX bracket expressions, the dot character matches a literal dot. For example, a.c matches "abc", etc., but [a.c] matches only "a", ".", or "c".
  • ? The question mark indicates zero or one occurrences of the preceding element. For example, colou?r matches both "color" and "colour".
    •     The asterisk indicates **zero or more** occurrences of the preceding element. For example, ab\*c matches "ac", "abc", "abbc", "abbbc", and so on.  

    •     The plus sign indicates **one or more** occurrences of the preceding element. For example, ab+c matches "abc", "abbc", "abbbc", and so on, but not "ac".
  • {n} The preceding item is matched exactly n times.
  • {min,} The preceding item is matched min or more times.
  • {min,max} The preceding item is matched at least min times, but not more than max times.
    • For example, a{3,5} matches only "aaa", "aaaa", and "aaaaa". This is not found in a few older instances of regexes. BRE mode requires {m,n}.

POSIX basic and extended

  • ^ Matches the starting position within the string. In line-based tools, it matches the starting position of any line.
  • $ Matches the ending position of the string or the position just before a string-ending newline. In line-based tools, it matches the ending position of any line.
  • \b word boundry. nt\b matches "nt" in "paint", but not in "pants".
  • \B word non-boundry. all\B matches "all" in "ally", but not in "wall".
  • [] A bracket expression. Matches a single character that is contained within the brackets. For example, [abc] matches "a", "b", or "c". [a-z] specifies a range which matches any lowercase letter from "a" to "z". These forms can be mixed: [abcx-z] matches "a", "b", "c", "x", "y", or "z", as does [a-cx-z]. The - character is treated as a literal character if it is the last or the first (after the ^, if present) character within the brackets: [abc-], [-abc]. Note that backslash escapes are not allowed. The ] character can be included in a bracket expression if it is the first (after the ^) character: []abc].
  • [^] Matches a single character that is not contained within the brackets. For example, [^abc] matches any character other than "a", "b", or "c". [^a-z] matches any single character that is not a lowercase letter from "a" to "z". Likewise, literal characters and ranges can be mixed.
  • () Defines a marked subexpression. The string matched within the parentheses can be recalled later (see the next entry, \n). A marked subexpression is also called a block or capturing group. BRE mode requires ( ).
  • \n Matches what the nth marked subexpression matched, where n is a digit from 1 to 9. This construct is vaguely defined in the POSIX.2 standard. Some tools allow referencing more than nine capturing groups.

Shortcuts

  • \d any digit: [0-9]
  • \D any non-digit: [^0-9]
  • \w any alphanumeric/underscore: [a-zA-Z0-9_]
  • \W any non-alphanumeric: [^a-zA-Z0-9\_]
  • \s whitespace(space, tab): [ \r\t\n\f]
  • \S non-whitespace: [^ \r\t\n\f]

e.g.

  • .at matches any three-character string ending with "at", including "hat", "cat", and "bat".
  • [hc]at matches "hat" and "cat".
  • [^b]at matches all strings matched by .at except "bat".
  • [^hc]at matches all strings matched by .at other than "hat" and "cat".
  • ^[hc]at matches "hat" and "cat", but only at the beginning of the string or line.
  • [hc]at$ matches "hat" and "cat", but only at the end of the string or line.
  • [.] matches any single character surrounded by "[" and "]" since the brackets are escaped, for example: "[a]" and "[b]".
  • s.* matches s followed by zero or more characters, for example: "s" and "saw" and "seed".