Determines whether a string contains a pattern (regular expression).

FoundPos := RegExMatch(Haystack, NeedleRegEx , &OutputVar, StartingPos)



Type: String

The string whose content is searched. This may contain binary zero.


Type: String

The pattern to search for, which is a Perl-compatible regular expression (PCRE). The pattern's options (if any) must be included at the beginning of the string followed by a close-parenthesis. For example, the pattern i)abc.*123 would turn on the case-insensitive option and search for "abc", followed by zero or more occurrences of any character, followed by "123". If there are no options, the ")" is optional; for example, )abc is equivalent to abc.

Although NeedleRegEx cannot contain binary zero, the pattern \x00 can be used to match a binary zero within Haystack.


Type: VarRef

Specify a reference to a variable in which to store a match object, which can be used to retrieve the position, length and value of the overall match and of each captured subpattern, if any are present.

If the pattern is not found (that is, if the function returns 0), this variable is made blank.


Type: Integer

If StartingPos is omitted, it defaults to 1 (the beginning of Haystack). Otherwise, specify 2 to start at the second character, 3 to start at the third, and so on. If StartingPos is beyond the length of Haystack, the search starts at the empty string that lies at the end of Haystack (which typically results in no match).

Specify a negative StartingPos to start at that position from the right. For example, -1 starts at the last character and -2 starts at the next-to-last character. If StartingPos tries to go beyond the left end of Haystack, all of Haystack is searched.

Specify 0 to start at the end of Haystack; i.e. the position to the right of the last character. This can be used with zero-width assertions such as (?<=a).

Regardless of the value of StartingPos, the return value is always relative to the first character of Haystack. For example, the position of "abc" in "123abc789" is always 4.

Return Value

Type: Integer

This function returns the position of the leftmost occurrence of NeedleRegEx in the string Haystack. Position 1 is the first character. Zero is returned if the pattern is not found.


Syntax errors: If the pattern contains a syntax error, an Error is thrown with a message in the following form: Compile error N at offset M: description. In that string, N is the PCRE error number, M is the position of the offending character inside the regular expression, and description is the text describing the error.

Execution errors: If an error occurs during the execution of the regular expression, an Error is thrown. The Extra property of the error object contains the PCRE error number. Although such errors are rare, the ones most likely to occur are "too many possible empty-string matches" (-22), "recursion too deep" (-21), and "reached match limit" (-8). If these happen, try to redesign the pattern to be more restrictive, such as replacing each * with a ?, +, or a limit like {0,3} wherever feasible.


See Options for modifiers such as i)abc, which turns off case-sensitivity in the pattern "abc".

Match Object (RegExMatchInfo)

If a match is found, an object containing information about the match is stored in OutputVar. This object has the following methods and properties:

Match.Pos, Match.Pos[N] or Match.Pos(N): Returns the position of the overall match or a captured subpattern.

Match.Len, Match.Len[N] or Match.Len(N): Returns the length of the overall match or a captured subpattern.

Match.Name[N] or Match.Name(N): Returns the name of the given subpattern, if it has one.

Match.Count: Returns the overall number of subpatterns (capturing groups), which is also the maximum value for N.

Match.Mark: Returns the NAME of the last encountered (*MARK:NAME), when applicable.

Match[] or Match[N]: Returns the overall match or a captured subpattern.

All of the above allow N to be any of the following:

Match.N: Shorthand for Match["N"], where N is any unquoted name or number which does not conflict with a defined property (listed above). For example, match.1 or match.Year.

The object also supports enumeration; that is, the for-loop is supported. Alternatively, use Loop Match.Count.


To search for a simple substring inside a larger string, use InStr because it is faster than RegExMatch.

To improve performance, the 100 most recently used regular expressions are kept cached in memory (in compiled form).

The study option (S) can sometimes improve the performance of a regular expression that is used many times (such as in a loop).


A subpattern may be given a name such as the word Year in the pattern (?P<Year>\d{4}). Such names may consist of up to 32 alphanumeric characters and underscores. Note that named subpatterns are also numbered, so if an unnamed subpattern occurs after "Year", it would be stored in OutputVar[2], not OutputVar[1].

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. Escaping can be avoided by using \Q...\E. For example: \QLiteral Text\E.

Within a regular expression, special characters such as tab and newline can be escaped with either an accent (`) or a backslash (\). For example, `t is the same as \t except when the x option is used.

To learn the basics of regular expressions (or refresh your memory of pattern syntax), see the RegEx Quick Reference.

AutoHotkey's regular expressions are implemented using Perl-compatible Regular Expressions (PCRE) from

Within an expression, the a ~= b can be used as shorthand for RegExMatch(a, b).

RegExReplace, RegEx Quick Reference, Regular Expression Callouts, InStr, SubStr, SetTitleMatchMode RegEx, Global matching and Grep (forum link)

Common sources of text data: FileRead, Download, A_Clipboard, GUI Edit controls


For general RegEx examples, see the RegEx Quick Reference.

Reports 4, which is the position where the match was found.

MsgBox RegExMatch("xxxabc123xyz", "abc.*xyz")

Reports 7 because the $ requires the match to be at the end.

MsgBox RegExMatch("abc123123", "123$")

Reports 1 because a match was achieved via the case-insensitive option.

MsgBox RegExMatch("abc123", "i)^ABC")

Reports 1 and stores "XYZ" in SubPat[1].

MsgBox RegExMatch("abcXYZ123", "abc(.*)123", &SubPat)

Reports 7 instead of 1 due to the starting position 2 instead of 1.

MsgBox RegExMatch("abc123abc456", "abc\d+",, 2)

Demonstrates the usage of the Match object.

FoundPos := RegExMatch("Michiganroad 72", "(.*) (?<nr>\d+)", &SubPat)
MsgBox SubPat.Count ": " SubPat[1] " " SubPat.Name[2] "="  ; Displays "2: Michiganroad nr=72"

Retrieves the extension of a file. Note that SplitPath can also be used for this, which is more reliable.

Path := "C:\Foo\Bar\Baz.txt"
RegExMatch(Path, "\w+$", &Extension)
MsgBox Extension[]  ; Reports "txt".

Similar to AutoHotkey v1's Transform Deref, the following function expands variable references and escape sequences contained inside other variables. Furthermore, this example shows how to find all matches in a string rather than stopping at the first match (similar to the g flag in JavaScript's RegEx).

var1 := "abc"
var2 := 123
MsgBox Deref("%var1%def%var2%")  ; Reports abcdef123.

    spo := 1
    out := ""
    while (fpo:=RegexMatch(Str, "(%(.*?)%)|``(.)", &m, spo))
        out .= SubStr(Str, spo, fpo-spo)
        spo := fpo + StrLen(m[0])
        if (m[1])
            out .= %m[2]%
        else switch (m[3])
            case "a": out .= "`a"
            case "b": out .= "`b"
            case "f": out .= "`f"
            case "n": out .= "`n"
            case "r": out .= "`r"
            case "t": out .= "`t"
            case "v": out .= "`v"
            default: out .= m[3]
    return out SubStr(Str, spo)