Programming Perl, 4th Edition

Scoped Pattern Modifiers

You may lexically scope the /i, /m, /s, /x, /d, /u, /a, /l, and /p modifiers within a portion of your pattern by inserting them (without the slash) between the ? and : of the grouping notation. If you say:

/Harry (?i:s) Truman/

it matches both “Harry S Truman” and “Harry s Truman”, whereas:

/Harry (?x: [A–Z] \.? \s )?Truman/

matches both “Harry S Truman” and “Harry S. Truman”, as well as “Harry Truman”; and:

/Harry (?ix: [A–Z] \.? \s )?Truman/

matches all five, by combining the /i and /x within the group.

You can also subtract modifiers from a scope with a minus sign:

/Harry (?x–i: [A–Z] \.? \s )?Truman/i

This matches any capitalization of the name—but if the middle initial is provided, it must be capitalized, since the /i applied to the overall pattern is suspended inside the scope.

When subtracting, you may only subtract /i, /m, /s, or /x. The /d, /u, /a, /l, and /p modifiers may only be added.

By omitting both colon and PATTERN, you can export modifier settings to an outer group, turning it into a scope. That is, you can selectively turn modifiers on and off for the grouping construct one level outside the modifiers’ parentheses, like so:

/(?i)foo/            # Equivalent to /foo/i
/foo((?–i)bar)/i     # "bar" must be lower case
/foo((?x–i)bar)/     # Enables /x and disables /i for "bar"

Note that the second and third examples create capture groups. If that wasn’t what you wanted, then you should have been using (?–i:bar) and (?x–i:bar), respectively.

Setting modifiers on a portion of ...