Syntax of Regular Expressions

• Introduction
• Simple matches
• Escape sequences
• Character classes
• Metacharacters - line separators
• Metacharacters - predefined classes
• Metacharacters - word boundaries
• Metacharacters - iterators
• Metacharacters - alternatives
• Metacharacters - subexpressions
• Metacharacters - backreferences
• Modifiers
• Lookahead/Lookbehind assertions
• PCRE regex engine
• Case changing

Introduction

Regular expressions are a widely-used method of specifying patterns of text to search for. Special metacharacters allows to specify, for instance, that a particular string you are looking for occurs at the beginning or end of a line, or contains n recurrences of a certain character.

SynWrite uses RegEx syntax, which is mostly compatible with PCRE, with minor additions and limitations.

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.

You 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.

Examples:

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.

Examples:

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.

Examples:

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.

Examples:

Metacharacters

Metacharacters are special characters which are the essence of Regular Expressions. There are different types of metacharacters, described below.

Metacharacters - line separators

Examples:

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 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 matches any character except newline characters, but if you switch "On" the modifier (?s), then '.' will match embedded line separators.

"^" is at the beginning of a input string, and, if modifier (?m) is "On", also immediately following any occurrence of \x0D\x0A or \x0A or \x0D (in Unicode also \x2028 or \x2029 or \x0B or \x0C or \x85). Note that there is no empty line within the sequence \x0D\x0A.

"$" is at the end of a input string, and, if modifier (?m) is "On", also immediately preceding any occurrence of \x0D\x0A or \x0A or \x0D (in Unicode also \x2028 or \x2029 or \x0B or \x0C or \x85). Note that there is no empty line within the sequence \x0D\x0A.

"." matchs any character, but if You switch "Off" modifier (?s) then "." doesn't match \x0D\x0A and \x0A and \x0D (in Unicode also \x2028 and \x2029 and \x0B and \x0C and \x85).

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

Metacharacters - predefined classes

You may use "lowercase" classes (\w, \d, \s, \h, \g) within custom character classes.

Examples:

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.

Metacharacters - iterators

Any item of a regular expression may be followed by another type of metacharacters - iterators. Using this metacharacters you can specify number of occurences of previous character, metacharacter or subexpression.

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 chew up more memory and slow down regular expression execution.

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

Examples:

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").

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|].

Examples:
foo(bar|foo) matchs strings 'foobar' or 'foofoo'.

Metacharacters - subexpressions

The bracketing construct ( ... ) may also be used to define regex subexpressions. Subexpressions are numbered based on the left to right order of their opening parenthesis. First subexpression has number 1, whole regex match has number 0.

Examples:

Subexpressions for search+replace

Text parts in round braces are taken as subexpressions.
Example: To swap two strings, when they are separated by a dash: "New York - USA", this can be solved like this. Result will be "USA - New York"

Metacharacters - backreferences

Metacharacters \1 through \9 are interpreted as backreferences. \n (where n is a digit) matches previously matched subexpression with index n.

Examples:

Modifiers

Modifiers are for changing behaviour of RegEx library. You can turn modifiers on/off. E.g., to turn modifier "i" on, insert such text at RegEx start: (?i), and to turn it off insert text (?-i).

Note: You may use modifiers in the middle of RegEx too. If this construction inlined into subexpression, then it effects only this subexpression.

Modifier "x":

The modifier "x" needs a little more explanation. It tells the RegEx library 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. - RegEx library 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.

Examples for modifier "i":

Example for modifier "s":

BEGIN
...Some.......
.....Lines....
...... Here...
END

(?#text)
A comment, the text is ignored. Note that RegEx library closes the comment as soon as it sees a ")", so there is no way to put a literal ")" in the comment.

Lookahead/Lookbehind assertions

Examples:

PCRE regex engine

The EControl regex syntax described above is used for lexers parsing. In searches from Find/Replace dialogs more powerful regex engine is used, PCRE. PCRE is almost 100% compatible with Perl regular expressions. The main differences between EControl and PCRE:

• PCRE allows to change the case on replacements, see help paragraph below.
• EControl doesn't support non-capturing groups which is a big drawback when you want to do some complex replacements.
• "r" modifier doesn't exist in PCRE.
• "g" modifier in EControl is same as "U" in PCRE.
• \g and \G don't exists in PCRE.
• \h and \H have another meaning in PCRE.
• PCRE recognizes only \x0D\x0A or \x0A or \x0D (depending of the file format) as newline characters when using "^", "$" and ".". Unicode line endings are not supported with ^ $ . in current implementation, but could be matched with \x metacharacter.
• EControl uses \z to match any newline character, while PCRE uses \z to match only at the end of text (same as \Z in EControl, but in PCRE \Z has another meaning - matches at the end of the string, also matches before a newline at the end of the string).

Case changing

Feature supported in PCRE engine only. You may change text case when doing replaces in editor. The following metacharacters are supported:

• \Un inserts the n-th backreference in uppercase;
• \Ln inserts it in lowercase;
• \Fn inserts it with the first character in uppercase and the remainder in lowercase;
• \In inserts it with the first letter of each word capitalized, and the other letters in lowercase.

For example, if we have text line "sOMe tExt hEre":