Modify Field Operations

Modify field operations modify a field value using regular expressions.

The modify_field operation can select a string of one length and replace it with a string of a different length.

Regular Expression Fields that modify_fields Supports lists the fields that modify_field supports in regular expressions. Multiple options can be specified, separated by a comma.

Important: You should be familiar with regular expressions before using the modify_field operation.

Table 1. Regular Expression Fields that modify_fields Supports
Field	Description
`match_default`	The matching of regular expressions proceeds without any modification.
`match_not_bob`	The matching expression "\A" cannot match the sub-sequence [first,first).
`match_not_eob`	The matching expressions "\z" and "\Z" cannot match the sub-sequence [last,last).
`match_not_bol`	The first character is not considered a beginning of line (bol); the matching expression "^" cannot match the sub-sequence [first,first).
`match_not_eol`	The last character is not considered an end of line (eol); the matching expression "$" cannot match the sub-sequence [last,last).
`match_not_bow`	The matching expression "\b" does not match as a beginning of word (bow); the matching expression "\b" cannot match the sub-sequence [first,first).
`match_not_eow`	The matching expression "\b" cannot match the sub-sequence [last,last).
`match_any`	If more than one match is possible, then any match is an acceptable result.
`match_not_null`	The matching expression cannot match an empty sequence.
`match_continuous`	The matching expression must match a sub-sequence that begins at the first character.
`match_partial`	Indicates that partial and full matches should be found.
`match_single_line`	Prevents ^ from matching after an embedded newline character (so that it only matches at the start of the text being matched). Prevents $ from matching before an embedded newline (so that it only matches at the end of the text being matched).
`match_prev_avail`	Finds a character in front of the target text to determine if it is at the beginning of a word. If set, `match_not_bol` and `match_not_bow` are ignored.
`match_not_dot_newline`	The matching expression "." does not match a newline character.
`match_not_dot_null`	The matching expression "." does not match a character null '\0'.
`match_posix`	The matching expression is matched according to the POSIX leftmost-longest rule.
`match_perl`	The matching expression is matched according to Perl matching rules.
`match_nosubs`	Matched results only contain information about the overall match, and not any sub-expressions.
`match_extra`	All available capture information is retained; if the matching expression is repeated then information about every repeat is available using match_results::captures() or sub_match_captures().
`format_default`	That replacing string is constructed using the rules used by the ECMAScript replace function.
`format_sed`	The matching expression is replaced with the replacing string using the rules used by the Unix sed utility in IEEE Std 1003.1-2001, Portable Operating System Interface (POSIX ), Shells and Utilities.
`format_perl`	The matching expression is replaced with the replacing string using the same rules as Perl 5.
`format_literal`	The matching expression is replaced with a literal copy of the replacing string.
`format_no_copy`	Sections of the search field that do match the matching expression, are not copied to the output string.
`format_first_only`	Only the first occurrence of the matching expression is replaced.
`format_all`	All syntax extensions are enabled.

For an example of the modify_field selective update configuration see the discussion about selective update examples.