Double Commander

2.14. Syntax of Regular Expressions


1. Introduction
2. Simple matches
3. Escape sequences
4. Character classes
5. Metacharacters
5.1. Metacharacters - line separators
5.2. Metacharacters - predefined classes
5.3. Metacharacters - word boundaries
5.4. Metacharacters - iterators
5.5. Metacharacters - alternatives
5.6. Metacharacters - subexpressions
5.7. Metacharacters - backreferences
6. Assertions (lookahead and lookbehind assertions)
7. Non-capturing groups
8. Atomic groups
9. Unicode categories
10. Modifiers

Double Commander uses the free library TRegExpr by Andrey Sorokin.

Most of the explanations are from the help file for this library.

1. Introduction

Regular Expressions are a widely-used method of specifying patterns of text to search for. Special characters (metacharacters) allow us to specify, for instance, that a particular string we are looking for occurs at the beginning or end of a line, or contains n recurrences of a certain character or group of characters.

Double Commander supports regular expressions in the following functions:

The TRegExp library supports two modes of operation: ANSI and Unicode. When searching in text files, Double Commander uses both (depending on the file encoding). When searching by name, Unicode is used.

2. Simple matches

Any single character matches itself, unless it is a metacharacter with a special meaning described below.

A series of characters matches that series of characters in the target string, so the pattern bluh would match bluh in the target string.

We can cause characters that normally function as metacharacters or escape sequences to be interpreted literally by "escaping" them by preceding them with a backslash \, for instance: metacharacter ^ match beginning of string, but \^ match character ^, \\ match \ and so on.

Here are some examples:

Example of simple match
matches string foobar
matches ^FooBarPtr

3. Escape sequences

Characters may be specified using a escape sequences syntax much like that used in C and Perl: \n matches a newline, \t a tab, etc.

More generally, \xnn, where nn is a string of hexadecimal digits, matches the character whose ASCII value is nn.

If you need wide (Unicode) character code, you can use \x{nnnn}, where nnnn – one or more hexadecimal digits.

Escape sequences
char with hex code nn
char with hex code nnnn (one byte for plain text and two bytes for Unicode)
tab (HT/TAB), same as \x09
newline (NL/LF), same as \x0a
carriage return (CR), same as \x0d
form feed (FF), same as \x0c
alarm (bell) (BEL), same as \x07
escape (ESC), same as \x1b

Here are some examples:

Example of escape sequences
matches foo bar (note space in the middle)
matches foobar predefined by tab

4. Character classes

You can specify a character class, by enclosing a list of characters in [], which will match any one character from the list.

If the first character after the [ is ^, the class matches any character not in the list.

Within a list, the - character is used to specify a range, so that a-z represents all characters between a and z, inclusive.

If you want - itself to be a member of a class, put it at the start or end of the list, or escape it with a backslash.

If you want ] you may place it at the start of list or escape it with a backslash.

Character classes
matches a, z and -
matches a, z and -
matches a, z and -
matches all twenty six small characters from a to z
matches any of \x10, \x11, \x12, \x13
matches any digit, - or t
matches any char from ]..a

Here are some examples:

Example of character classes
finds strings foobar, foober etc. but not foobbr, foobcr etc.
find strings foobbr, foobcr etc. but not foobar, foober etc.

5. Metacharacters

Metacharacters are special characters which are the essence of Regular Expressions.

There are different types of metacharacters, described below.

5.1. Metacharacters - line separators

Some expressions help to detect line separation.

Line separators
start of line
end of line
start of text
end of text
any character in line

Here are some examples:

Example with line separators
matches string foobar only if it's at the beginning of line
matches string foobar only if it's at the end of line
matches string foobar only if it's the only string in line
matches strings like foobar, foobbr, foob1r and so on

The ^ metacharacter by default is only guaranteed to match at the beginning of the input string/text, the $ metacharacter only at the end. Embedded line separators will not be matched by ^ or $.

You may, however, wish to treat a string as a multi-line buffer, such that the ^ will match after any line separator within the string, and $ will match before any line separator. You can do this by switching On the modifier m.

The \A and \Z are just like ^ and $, except that they won't match multiple times when the modifier m is used, while ^ and $ will match at every internal line separator.

The . metacharacter by default matches any character, but if you switch Off the modifier s, then . won't match embedded line separators.

TRegExpr works with line separators as recommended at Unicode technical standards (Technical Standard #18):

Note that ^.*$ (an empty line pattern) does not match the empty string within the sequence \x0D\x0A, but matches the empty string within the sequence \x0A\x0D.

5.2. Metacharacters - predefined classes

Some expressions help to detect group of characters.

Predefined classes
an alphanumeric character (including _), i.e. [0-9A-Za-z_]
a nonalphanumeric
a numeric character
a non-numeric
any space (same as [ \t\n\r\f])
a non space

You may use \w, \d and \s within custom character classes.

Here are some examples:

Example of predefined classes
matches strings like foob1r, foob6r and so on but not foobar, foobbr and so on
matches strings like foobar, foob r, foobbr and so on but not foob=r and so on

5.3. Metacharacters - word boundaries

A word boundary (\b) is 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.

Word boundaries
match a word boundary
match a non-(word boundary)

5.4. Metacharacters - iterators

Any item of a regular expression may be followed by another type of metacharacters – iterators.

Using these metacharacters you can specify the number of occurrences of the previous character, metacharacter or subexpression.

zero or more ("greedy"), similar to {0,}
one or more ("greedy"), similar to {1,}
zero or one ("greedy"), similar to {0,1}
exactly n times ("greedy")
at least n times ("greedy")
at least n but not more than m times ("greedy")
zero or more ("non-greedy"), similar to {0,}?
one or more ("non-greedy"), similar to {1,}?
zero or one ("non-greedy"), similar to {0,1}?
exactly n times ("non-greedy")
at least n times ("non-greedy")
at least n but not more than m times ("non-greedy")

So, digits in curly brackets of the form {n,m}, specify the minimum number of times to match the item n and the maximum m.

The form {n} is equivalent to {n,n} and matches exactly n times.

The form {n,} matches n or more times.

There is no limit to the size of n or m, but large numbers will slow down execution and chew up more memory.

If a curly bracket occurs in any other context, it is treated as a regular character.

Here are some examples:

Example of iterators
matches strings like foobar, foobalkjdflkj9r and foobr
matches strings like foobar, foobalkjdflkj9r but not foobr
matches strings like foobar, foobbr and foobr but not foobalkj9r
matches the string foobaar
matches strings like foobaar, foobaaar, foobaaaar etc.
matches strings like foobaar, or foobaaar but not foobaaaar

A little explanation about "greediness".

"Greedy" takes as many as possible, "non-greedy" takes as few as possible.

For example, b+ and b* applied to string abbbbc return bbbb, b+? returns b, b*? returns empty string, b{2,3}? returns bb, b{2,3} returns bbb.

You can switch all iterators into "non-greedy" mode (see the modifier g).

5.5. Metacharacters - alternatives

You can specify a series of alternatives for a pattern using | to separate them, so that fee|fie|foe will match any of fee, fie, or foe in the target string (as would f(e|i|o)e).

The first alternative includes everything from the last pattern delimiter ((, [, or the beginning of the pattern) up to the first |, and the last alternative contains everything from the last | to the next pattern delimiter.

For this reason, it's common practice to include alternatives in parentheses, to minimize confusion about where they start and end.

Alternatives are tried from left to right, so the first alternative found for which the entire expression matches, is the one that is chosen.

This means that alternatives are not necessarily greedy.

For example: when matching foo|foot against barefoot, only the foo part will match, as that is the first alternative tried, and it successfully matches the target string. (This might not seem important, but it is important when you are capturing matched text using parentheses.)

Also remember that | is interpreted as a literal within square brackets, so if you write [fee|fie|foe] you're really only matching [feio|].


Example of alternatives
matches strings foobar or foofoo

5.6. Metacharacters - subexpressions

The bracketing construct ( ... ) may also be used for define regular expression subexpressions.

After search you can call any subexpression, also you can use subexpressions as masks.

Subexpressions are numbered based on the left to right order of their opening parenthesis.

First subexpression has number 1, up to 90 subexpressions are supported (whole regular expression match has number 0 – you can substitute it as $0 or $&).

Here are some examples:

matches strings which contain 8, 9 or 10 instances of the foobar
matches foob0r, foob1r , foobar, foobaar, foobaar etc.

Notes about "Replace with" templates:


Let's invert date 21.01.2018 > 2018.01.21:
  search for: (\d{2})\.(\d{2})\.(\d{4})
  replace with: $3.$2.$1

5.7. Metacharacters - backreferences

Metacharacters \1 through \9 are interpreted as backreferences. \n matches previously matched subexpression n.

Here are some examples:

Examples of backreferences
matches aaaa and cc
also match abab and 123123
matches "13" (in double quotes), or '4' (in single quotes) or 77 (without quotes) etc

6. Assertions (lookahead and lookbehind assertions)

Positive lookahead assertion: foo(?=bar) matches foo only before bar, and bar is excluded from the match.

Negative lookahead assertion: foo(?!bar) matches foo only if it’s not followed by bar.

Positive lookbehind assertion: (?<=foo)bar matches bar only after foo, and foo is excluded from the match.

Negative lookbehind assertion: (?<!foo)bar matches bar only if it’s not prefixed with foo.


7. Non-capturing groups

Syntax: (?:expr).

Such groups do not have the "index" and are invisible for backreferences. Non-capturing groups are used when you want to group a subexpression, but you do not want to save it as a matched/captured portion of the string. The use of non-capturing groups allows to speed up the work of a regular expression.

Non-capturing groups
in matches https and
in matches only

8. Atomic groups

Syntax: (?>expr|expr|...).

Atomic groups are special case of non-capturing groups: this grouping disables backtracking for a parenthesized group if part of the pattern has already been found. Atomic group works faster, useful when optimizing groups with many different expressions.

For example, a(bc|b)c matches abcc and abc, a(?>bc|b)c matches abcc but not abc because the engine is forbidden from backtracking and try with setting the group as b.

9. Unicode categories

Unicode standard has names for character categories. These are 2-letter strings. For example Lu is uppercase letters, Ll is lowercase letters. And 1-letter bigger category L is all letters.

Unicode categories
Uppercase Letter
Lowercase Letter
Titlecase Letter
Modifier Letter
Other Letter
Non-Spacing Mark
Spacing Combining Mark
Enclosing Mark
Decimal Digit Number
Letter Number
Other Number
Connector Punctuation
Dash Punctuation
Open Punctuation
Close Punctuation
Initial Punctuation
Final Punctuation
Other Punctuation
Math Symbol
Currency Symbol
Modifier Symbol
Other Symbol
Space Separator
Line Separator
Paragraph Separator
Private Use

Meta-character \p denotes one Unicode char of specified category. Syntaxes: \pL and \p{L} for 1-letter name, \p{Lu} for 2-letter names.

Meta-character \P is inverted, it denotes one Unicode char not in the specified category.

These meta-characters are supported withing character classes too.

10. Modifiers

Syntax for one modifier: (?i) to turn on, and (?-i) to turn off. Many modifiers are allowed like this: (?msgxr-imsgxr).

Modifiers are for changing behaviour of regular expressions. Modifier affects only that part of regular expression that follows (?imsgxr-imsgxr) operator.

Any of these modifiers may be embedded within the regular expression itself. If modifier is inlined into subexpression, then it effects only into this subexpression.

Do case-insensitive pattern matching (using installed in your system locale settings). Disabled by default.
Treat string as multiple lines. That is, change ^ and $ from matching at only the very start or end of the string to the start or end of any line anywhere within the string, see also Line separators. Disabled by default.
Treat string as single line. That is, change . to match any character whatsoever, even a line separator (see also Line separators), which it normally would not match. Enabled by default.
Non standard modifier. Switching it Off you'll switch all following operators into non-greedy mode (by default this modifier is On). So, if modifier g is Off then + works as +?, * as *? and so on.
Extend your pattern's legibility by permitting whitespace and comments (see explanation below). Disabled by default.
Non-standard modifier. If is set then range а-я additional include Russian letter ё, А-Я additional include Ё, and а-Я include all Russian alphabet. Enabled by default.
(?#text): A comment, the text is ignored. Note that TRegExpr closes the comment as soon as it sees a ), so there is no way to put a literal ) in the comment.

Here are some examples:

Examples of Perl extensions
matches Saint-petersburg and Saint-Petersburg
matches Saint-Petersburg but not Saint-petersburg
matches Saint-petersburg and saint-petersburg
matches saint-Petersburg but not saint-petersburg

The modifier x itself needs a little more explanation.

It tells to ignore whitespace that is neither backslashed nor within a character class.

You can use this to break up your regular expression into (slightly) more readable parts.

The # character is also treated as a metacharacter introducing a comment, for example:

  (abc) # comment 1
    |   # You can use spaces to format r.e. - TRegExpr ignores it
  (efg) # comment 2

This also means that if you want real whitespace or # characters in the pattern (outside a character class, where they are unaffected by x), that you'll either have to escape them or encode them using octal or hex escapes.

Taken together, these features go a long way towards making regular expressions text more readable.

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