RegexReplace (String function)

From m204wiki
Jump to navigation Jump to search

Replace regex match(es) (String class)

The RegexReplace intrinsic function searches a given string for matches of a regular expression, and replaces matches with, or according to, a specified replacement string. The function stops after the first match and replace, or it can continue searching and replacing until no more matches are found.

Matches are obtained according to the rules of regular expression matching.


%outString = string:RegexReplace( regex, replacement, [Options= string]) Throws InvalidRegex

Syntax terms

%outString A string set to the value of method object string with each matched substring replaced by the value of replacement.
string The method object string, within which matches for regex are sought.
regex A string that is interpreted as a regular expression and that is applied to the method object string to find the one or more substrings matched by regex
replacement The string that replaces the substrings of string that regex matches. Except when the A option is specified (as described below for the Options argument), you can include markers in the replacement value to indicate where to insert corresponding captured strings — strings matched by capturing groups (parenthesized subexpressions) in regex, if any.

As in Perl, these markers are in the form $n, where n is the number of the capture group, and 1 is the number of the first capture group. n must not be 0 or contain more than 9 digits. If a capturing group makes no matches (is positional, for example), or if there was no nth capture group corresponding to the $n marker in a replacement string, the (literal) value of $n is used in the replacement string instead of the empty string. xxx$1 is an example of a valid replacement string, and $0yyy is an example of an invalid one. Or you can use the format $mn, where m is one of the following modifiers:

U or u Specifies that the specified captured string should be uppercased when inserted.
L or l Indicates that the captured string should be lowercased when inserted.

The only characters you can escape in a replacement string are dollar sign ($), backslash (\), and the digits 0 through 9. So only these escapes are respected: \\, \$, and \0 through \9. No other escapes are allowed in a replacement string — this includes "shorthand" escapes like \d — and an "unaccompanied" backslash (\) is an error. For example, since the scan for the number that accompanies the meta-$ stops at the first non-numeric, you use 1$1\2 to indicate that the first captured string should go between the numbers 1 and 2 in the replacement string.

An invalid replacement string results in request cancellation.

Options This optional, name required, parameter is a string of single-letter options, which may be specified in uppercase or lowercase, in any combination, and blank separated or not, as you prefer. For more information about these options, see Common regex options.

Usage notes

  • It is strongly recommended that you protect your environment from regular expression processing demands on PDL and STBL space by setting, say, UTABLE LPDLST 3000 and UTABLE LSTBL 9000. See SOUL programming considerations.
  • Within a regular expression, characters enclosed by a pair of unescaped parentheses form a "subexpression." A subexpression is a capturing group if the opening parenthesis is not followed by a question mark (?). A capturing group that is nested within a non-capturing subexpression is still a capturing group.
  • In Perl, $n markers ($1, for example) enclosed in single quotes are treated as literals instead of as "that which was captured by the first capturing parentheses." RegexReplace uses the A option of the Options argument for this purpose.
  • Matching of regex may "succeed" but yet match no characters. For example, a quantifier like ? is allowed by definition to match no characters, though it tries to match one. RegexReplace honors such a zero-length match by substituting the specified replacement string at the current position. If the global option is in effect, the regex is then applied again one position to the right in the input string, and again, until the end of the string. The regex 9? globally applied to the string abc with a comma-comma (,,) replacement string results in this output string: ,,a,,b,,c,,.
  • For information about additional methods that support regular expressions, see Regex processing.


  1. In the following example, the regex (5.) is applied repeatedly (global option) to the string 5A5B5C5D5E to replace the uppercase letters with their lowercase counterparts. The $L1 replacement value makes the replacement string equal to whatever is matched by the capturing group, (5.), in the regex (the L causes the lowercase versions of the captured letters to be used).

    begin %regex longstring %inStr longstring %replacement longstring %outStr longstring %opt string len 10 %inStr='5A5B5C5D5E' %regex='(5.)' %replacement='$L1' %opt='g' %outStr = %inStr:regexReplace(%regex, %replacement, options=%opt) printText OutputString: '{%outStr}' end

    The example result is:

    OutputString: '5a5b5c5d5eBold text'

    The non-capturing regex 5. matches and replaces the same substrings as the capturing group (5.), but (5.) is used above to take advantage of the self-referring marker for the replacement string, $L1, which is valid only for capturing groups.

  2. Say you want to supply end tags to items of the form <img foo="bar">, converting them to <img foo="bar"></img>. You decide to use the following regex to capture img tags that have attributes:

    (<img .*>)

    And you use the following replacement string to replace the captured string with the captured string plus an appended </img>:


    However, if the regex above is applied to the string <body><img src="foo" width="24"></body>, the end tag </img> is not inserted after the first closing angle bracket (>) after 24 as you want. Instead, the matched string greedily extends to the second closing angle bracket, and the tag </img> is positioned at the end:

    <body><img src="foo" width="24"></body></img>

    One remedy for this situation is to use the following regex, which employs a negated character class to match non-closing-bracket characters:

    (<img [^>]*>)

    This regex does not extend beyond the first closing angle bracket in the target input string, and the resulting output string is:

    <body><img src="foo" width="24"></img></body>

See also