Regualar Expressions

# Regular Expressions

Because regular expressions that may be used in TrEd are the same as regular expressions that are defined in Perl, the following text is an extraction from Perl documentation.

The following metacharacters have their standard *egrep*-ish meanings:

`\`

  Quote the next metacharacter

`^`

  Match the beginning of the string

`.`

  Match any character

`$`

  Match the end of the string

`|`

  Alternation

`()`

  Grouping

`[]`

  Character class

The following standard quantifiers are recognized:

`*`

  Match 0 or more times

`+`

  Match 1 or more times

`?`

  Match 1 or 0 times

`{n}`

  Match exactly n times

`{n,}`

  Match at least n times

`{n,m}`

  Match at least n but not more than m times

(If a curly bracket occurs in any other context, it is treated as a regular character.) The "*" modifier is equivalent to `{0,}', the "+" modifier to `{1,}', and the "?" modifier to `{0,1}'. n and m are limited to integral values less than 65536.

By default, a quantified subpattern is "greedy", that is, it will match as many times as possible (given a particular starting location) while still allowing the rest of the pattern to match. If you want it to match the minimum number of times possible, follow the quantifier with a "?". Note that the meanings don't change, just the "greediness":

*?

  Match 0 or more times

`+?`

  Match 1 or more times

`??`

  Match 0 or 1 time

`{n}?`

  Match exactly n times

`{n,}?`

  Match at least n times

`{n,m}?`

  Match at least n but not more than m times

Because patterns are processed as double quoted strings, the following also work:

`\t`

  tab (HT, TAB)

`\n`

  newline (LF, NL)

`\r`

  return (CR)

`\f`

  form feed (FF)

`\a`

  alarm (bell) (BEL)

`\e`

  escape (think troff) (ESC)

`\033`

  octal char (think of a PDP-11)

`\x1B`

  hex char

`\c[`

  control char

`\l`

  lowercase next char

`\u`

  uppercase next char

`\L`

  lowercase till \E

`\U`

  uppercase till \E

`\E`

  end case modification

`\Q`

  quote regexp metacharacters till \E

If `UseCzechLocales` is set to non-zero value in TrEd's configuration, the list of alphabetic characters generated by `\l',`\u',`\L',`\U' is taken from the Czech locale. In addition, the following characters are defined:

`\w`

  Match a "word" character (alphanumeric plus "_")

`\W`

  Match a non-word character

`\s`

  Match a whitespace character

`\S`

  Match a non-whitespace character

`\d`

  Match a digit character

`\D`

  Match a non-digit character

Note that `\w' matches a single alphanumeric character, not a whole word. To match a word you'd need to say `\w+'. If `UseCzechLocales` is set to non-zero value in TrEd's configuration, the list of alphabetic characters generated by `\w' is taken from the Czech locale. You may use `\w', `\W', `\s', `\S', `\d', and `\D' within character classes (though not as either end of a range).

The following zero-width assertions are defined:

`\b`

  Match a word boundary

`\B`

  Match a non-(word boundary)

`\A`

  Match at only beginning of string

`\Z`

  Match at only end of string (or before newline at the end)

A word boundary (`\b') is defined as a spot between two characters that has a `\w' on one side of it and a `\W' on the other side of it (in either order), counting the imaginary characters off the beginning and end of the string as matching a `\W'. (Within character classes `\b' represents backspace rather than a word boundary.) The `\A' and `\Z' are just like "^" and "$".

When the bracketing construct ``( ... )`' is used, `\<digit>` matches the digit'th substring. If you want to use parentheses to delimit a subpattern (e.g., a set of alternatives) without saving it as a subpattern, follow the `(` with a `?:`.

You may have as many parentheses as you wish. Within the pattern, `\10`, `\11`, etc. refer back to substrings if there have been at least that many left parentheses before the backreference. Otherwise (for backward compatibility) `\10` is the same as `\010`, a backspace, and `\11` the same as `\011`, a tab. And so on. (`\1` through `\9` are always backreferences.)

`$+` returns whatever the last bracket match matched. `$&` returns the entire matched string. (`$0` used to return the same thing, but not any more.) ``$``' returns everything before the matched string. ``$'`' returns everything after the matched string.

TrEd defines a consistent extension syntax for regular expressions. The syntax is a pair of parentheses with a question mark as the first thing within the parentheses (this was a syntax error in older versions of Perl). The character after the question mark gives the function of the extension. Several extensions are already supported:

`(?#text)`

  A comment. The text is ignored. If the ``/x`' switch is used to enable whitespace formatting, a simple ``#`' will suffice.

`(?:regexp)`

  This groups things like "`()`" but doesn't make backreferences like "`()`" does.

(?!regexp)

  A zero-width negative lookahead assertion. For example ``foo(?!bar)`' matches any occurrence of "foo" that isn't followed by "bar". Note however that lookahead and lookbehind are NOT the same thing. You cannot use this for lookbehind: `/(?!foo)bar/' will not find an occurrence of "bar" that is preceded by something which is not "foo". That's because the `(?!foo)' is just saying that the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will match. You would have to do something like ``(?!foo)...bar`' for that. We say "like" because there's the case of your "bar" not having three characters before it. You could cover that this way: ``(?:(?!foo)...|^..?)bar`'.

(?ix)

  One or more embedded pattern-match modifiers. If you need a case insensitive pattern you only need to include ``(?i)`' at the front of the pattern.

  x

    Use extended regular expressions.

  i

    Do case-insensitive pattern matching.