Match anywhere: By default, a regular expression matches a substring anywhere inside the string to be searched. For example, the regular expression abc matches abc123, 123abc, and 123abcxyz. To require the match to occur only at the beginning or end, use an anchor.
Escaped characters: Most characters like abc123 can be used literally inside a regular expression. However, the characters \.*?+[{|()^$ must be preceded by a backslash to be seen as literal. For example, \. is a literal period and \\ is a literal backslash.
Case-sensitive: By default, regular expressions are case-sensitive. This can be changed via the "i" option. For example: i)abc searches for "abc" without regard to case. See options for other modifiers.
| . | A dot matches any single character (except newline: `r and `n). For example, ab. matches abc and abz and ab_ |
| * | An asterisk matches zero or more of the preceding character, set, or subpattern. For example, a* matches ab and aaab. It also matches at the very beginning of any string that contains no "a" at all. Wildcard: The pattern .* is one of the most permissive because it matches zero or more occurrences of any character (except newline: `r and `n). For example, abc.*123 matches abcAnything123 as well as abc123. |
| ? | A question mark matches zero or one of the preceding character, set, or subpattern. Think of this as "the preceding item is optional". For example, colou?r matches both color and colour because the "u" is optional. |
| + | A plus sign matches one or more of the preceding character, set, or subpattern. For example a+ matches ab and aaab. But unlike a* and a?, the pattern a+ does not match at the beginning of strings that lack an "a" character. |
| {min,max} | Matches between min and max occurrences of the preceding character, set, or subpattern. For example, a{1,2} matches ab but only the first two a's in aaab. Also, {3} means exactly 3 occurrences, and {3,} means 3 or more occurrences. Note: The specified numbers must be less than 65536, and the first must be less than or equal to the second. |
| [...] | Sets: The square brackets enclose a list or range of characters (or both). For example, [abc] means "any single character that is either a, b or c". Using a dash in between creates a range; for example, [a-z] means "any single character that is between lowercase a and z (inclusive)". A character set may be followed by *, ?, +, or {min,max}. For example, [0-9]+ matches one or more occurrence of any digit; thus it matches xyz123 but not abcxyz. Sets and ranges may be combined; for example [a-zA-Z0-9_] means "any single character that is alphanumeric or underscore". |
| [^...] | Matches any single character that is not in the set. For example, [^/]* matches zero or more occurrences of any character that is not a forward-slash, such as http://. Similarly, [^0-9xyz] matches any single character that isn't a digit and isn't the letter x, y, or z. |
| \d | Matches any single digit (equivalent to the set [0-9]). Conversely, capital \D means "any non-digit". This and the other two below can also be used inside a set; for example, [\d.-] means "any single digit, period, or minus sign". |
| \s | Matches any single whitespace character, mainly space, tab, and newline (`r and `n). Conversely, capital \S means "any non-whitespace character". |
| \w | Matches any single "word" character, namely alphanumeric or underscore. This is equivalent to [a-zA-Z0-9_]. Conversely, capital \W means "any non-word character". |
| ^ $ |
Circumflex (^) and dollar sign ($) are called anchors because they don't consume any characters but instead tie the pattern to the beginning or end of the string being searched. ^ may appear at the beginning of a pattern to require the match to occur at the very beginning of a line. For example, ^abc matches abc123 but not 123abc. $ may appear at the end of a pattern to require the match to occur at the very end of a line. For example, abc$ matches 123abc but not abc123. The two anchors may be combined. For example, ^abc$ matches only abc (i.e. there must be no other characters before or after it). If the text being searched contains multiple lines, the anchors can be made to apply to each line rather than the text as a whole by means of the "m" option. For example, m)^abc$ matches 123`r`nabc`r`n789. But without the "m" option, it wouldn't match. |
| \b | Word boundary (\b) is like an anchor because it doesn't consume any characters. Instead, it requires the current character's status as a word character (\w) to be the opposite of the previous character's. It is typically used to avoid accidentally matching a word that appears inside some other word. For example, \bcat\b doesn't match catfish, but it matches cat regardless what punctuation and whitespace surrounds it. Capital \B is the opposite: it requires that the current character not be at a word boundary. |
| | | The vertical bar separates two or more alternatives. A match occurs if any of the alternatives is satisfied. For example, gray|grey matches both gray and grey. Similarly, the pattern gr(a|e)y does the same thing with the help of the parentheses described below. |
| (...) | Items enclosed in parentheses are most commonly used to:
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| \t \r etc. |
These escape sequences stand for special characters. The most common ones are \t (tab), \r (carriage return), and \n (linefeed). In AutoHotkey, an accent (`) may optionally be used in place of the backslash in these cases. PCRE also supports other escape sequences such as \xhh (in which hh is the hex code of any ANSI character between 00 and FF). |
Greed: By default, *, ?, +, and {min,max} are greedy because they consume all characters up through the last possible one that still satisfies the entire pattern. To instead have them stop at the first possible character, follow them with a question mark. For example, the pattern <.+> (which lacks a question mark) means: "search for a <, followed by one or more of any character, followed by a >". To stop this pattern from matching the entire string <em>text</em>, append a question mark to the plus sign: <.+?>. This causes the match to stop at the first '>' and thus it matches only the first tag <em>.
Look-ahead and look-behind assertions: The groups (?=...), (?!...), (?<=...), and (?<!...) are called assertions because they demand a condition to be met but don't consume any characters. For example, abc(?=.*xyz) is a look-ahead assertion that requires the string xyz to exist somewhere to the right of the string abc (if it doesn't, the entire pattern is not considered a match). (?=...) is called a positive look-ahead because it requires that the specified pattern exist. Conversely, (?!...) is a negative look-ahead because it requires that the specified pattern not exist. Similarly, (?<=...) and (?<!...) are positive and negative look-behinds (respectively) because they look to the left of the current position rather than the right. Look-behinds are more limited than look-aheads because they support only literal characters and alternation (the vertical bar).
Related: Regular expressions are supported by RegExMatch(), RegExReplace(), and SetTitleMatchMode.
Final note: Although this page touches upon most of the commonly-used RegEx features, there are quite a few other features you may want to explore such as conditional subpatterns.