Pattern Matching - Grouping and Delimiting

Anchors

• By default, matching occurs anywhere in the string
• Provide matching the position of a pattern
• use anchors to match the position or location of a pattern
• Caret (^) indicates the start of a string, matching the beginning of a string
  • distinct from the caret in character class
  • redundant when using the match() method
• Dollar sign ($) indicates the end of a string

Start Anchor Examples

Pattern	String
^[Bb]erkeley	UC Berkeley	no match
^[Bb]erkeley	berkeley	match
^[Bb]erkeley	Berkeley, Ca	match
^[Bb]erkeley	UC Berkley	no match (misspelling)

Anchors Examples

• $ will only match strings that contain Berkeley

Pattern	String
^[Bb]erkeley$	UC Berkeley	no match
^[Bb]erkeley$	berkeley	match
^[Bb]erkeley$	Berkeley, Ca	no match
^[Bb]erkeley$	UC Berkley	no match (misspelling)

Grouping — Alternative Patterns

• Pipe or vertical bar character (

  ) separates multiple patterns

  • Acts like an or operator

    • pattern1

      pattern2

      pattern3

  • Match is made if any of the pattern have a match
  • pattern matching goes in sequence with groups

Or Example

Pattern	String
software|hardware	software	no match
software|hardware	hardware	no match
software|hardware	dinnerware hardware	match
software|hardware	firmware	no match

Grouping Patterns

• parenthesis allow for grouping of patterns
• treats a series of patterns as one unit (repetition qualifiers)

Grouping

(...)
# starts and ends a group of regex patterns
(?P<name>...)
# starts and ends a named group name of regex patterns

Grouping Example

Pattern	String
(soft|hard)ware	software	match
(soft|hard)ware	hardware	match
(soft|hard)ware	dinnerware hardware	match
(soft|hard)ware	firmware	no match