CharFilters

CharFilter is a component that pre-processes input characters.

CharFilters can be chained like Token Filters and placed in front of a Tokenizer. CharFilters can add, change, or remove characters while preserving the original character offsets to support features like highlighting.

solr.MappingCharFilterFactory

This filter creates org.apache.lucene.analysis.MappingCharFilter, which can be used for changing one string to another (for example, for normalizing é to e.).

This filter requires specifying a mapping argument, which is the path and name of a file containing the mappings to perform.

Example:

  • With name

  • With class name (legacy)

<analyzer>
  <charFilter name="mapping" mapping="mapping-FoldToASCII.txt"/>
  <tokenizer ...>
  [...]
</analyzer>
<analyzer>
  <charFilter class="solr.MappingCharFilterFactory" mapping="mapping-FoldToASCII.txt"/>
  <tokenizer ...>
  [...]
</analyzer>

Mapping file syntax:

  • Comment lines beginning with a hash mark (#), as well as blank lines, are ignored.

  • Each non-comment, non-blank line consists of a mapping of the form: "source" => "target"

    • Double-quoted source string, optional whitespace, an arrow (=>), optional whitespace, double-quoted target string.

  • Trailing comments on mapping lines are not allowed.

  • The source string must contain at least one character, but the target string may be empty.

  • The following character escape sequences are recognized within source and target strings:

    Escape Sequence Resulting Character (ECMA-48 alias) Unicode Character Example Mapping Line

    \\

    \

    U+005C

    "\\" => "/"

    \"

    "

    U+0022

    "\"and\"" => "'and'"

    \b

    backspace (BS)

    U+0008

    "\b" => " "

    \t

    tab (HT)

    U+0009

    "\t" => ","

    \n

    newline (LF)

    U+000A

    "\n" => "<br>"

    \f

    form feed (FF)

    U+000C

    "\f" => "\n"

    \r

    carriage return (CR)

    U+000D

    "\r" => "/carriage-return/"

    \uXXXX

    Unicode char referenced by the 4 hex digits

    U+XXXX

    "\uFEFF" => ""

    • A backslash followed by any other character is interpreted as if the character were present without the backslash.

solr.HTMLStripCharFilterFactory

This filter creates org.apache.solr.analysis.HTMLStripCharFilter. This CharFilter strips HTML from the input stream and passes the result to another CharFilter or a Tokenizer.

This filter:

  • Removes HTML/XML tags while preserving other content.

  • Removes attributes within tags and supports optional attribute quoting.

  • Removes XML processing instructions, such as: <?foo bar?>

  • Removes XML comments.

  • Removes XML elements starting with <!>.

  • Removes contents of <script> and <style> elements.

  • Handles XML comments inside these elements (normal comment processing will not always work).

  • Replaces numeric character entities references like &#65; or &#x7f; with the corresponding character.

  • The terminating ';' is optional if the entity reference at the end of the input; otherwise the terminating ';' is mandatory, to avoid false matches on something like "Alpha&Omega Corp".

  • Replaces all named character entity references with the corresponding character.

  • &nbsp; is replaced with a space instead of the 0xa0 character.

  • Newlines are substituted for block-level elements.

  • <CDATA> sections are recognized.

  • Inline tags, such as <b>, <i>, or <span> will be removed.

  • Uppercase character entities like quot, gt, lt and amp are recognized and handled as lowercase.

The input need not be an HTML document. The filter removes only constructs that look like HTML. If the input doesn’t include anything that looks like HTML, the filter won’t remove any input.

The table below presents examples of HTML stripping.

Input Output

my <a href="www.foo.bar">link</a>

my link

<br>hello<!--comment-->

hello

hello<script><!-- f('<!--internal--></script>'); --></script>

hello

if a<b then print a;

if a<b then print a;

hello <td height=22 nowrap align="left">

hello

a<b &#65 Alpha&Omega Ω

a<b A Alpha&Omega Ω

Example:

  • With name

  • With class name (legacy)

<analyzer>
  <charFilter name="htmlStrip"/>
  <tokenizer ...>
  [...]
</analyzer>
<analyzer>
  <charFilter class="solr.HTMLStripCharFilterFactory"/>
  <tokenizer ...>
  [...]
</analyzer>

solr.ICUNormalizer2CharFilterFactory

This filter performs pre-tokenization Unicode normalization using ICU4J.

Arguments:

form

Optional

Default: nfkc_cf

A Unicode Normalization Form, one of nfc, nfkc, or nfkc_cf.

mode

Optional

Default: compose

Either compose or decompose. Default is compose. Use decompose with name="nfc" or name="nfkc" to get NFD or NFKD, respectively.

filter

Optional

Default: []

A UnicodeSet pattern. Codepoints outside the set are always left unchanged. Default is [], as a null set, no filtering (all codepoints are subject to normalization).

Example:

  • With name

  • With class name (legacy)

<analyzer>
  <charFilter name="icuNormalizer2"/>
  <tokenizer ...>
  [...]
</analyzer>
<analyzer>
  <charFilter class="solr.ICUNormalizer2CharFilterFactory"/>
  <tokenizer ...>
  [...]
</analyzer>

solr.PatternReplaceCharFilterFactory

This filter uses regular expressions to replace or change character patterns.

Arguments:

pattern

Required

Default: none

The regular expression pattern to apply to the incoming text.

replacement

Required

Default: none

The text to use to replace matching patterns.

You can configure this filter in the schema like this:

  • With name

  • With class name (legacy)

<analyzer>
  <charFilter name="patternReplace"
             pattern="([nN][oO]\.)\s*(\d+)" replacement="$1$2"/>
  <tokenizer ...>
  [...]
</analyzer>
<analyzer>
  <charFilter class="solr.PatternReplaceCharFilterFactory"
             pattern="([nN][oO]\.)\s*(\d+)" replacement="$1$2"/>
  <tokenizer ...>
  [...]
</analyzer>

The table below presents examples of regex-based pattern replacement:

Input Pattern Replacement Output Description

see-ing looking

(\w+)(ing)

$1

see-ing look

Removes "ing" from the end of word.

see-ing looking

(\w+)ing

$1

see-ing look

Same as above. 2nd parentheses can be omitted.

No.1 NO. no. 543

[nN][oO]\.\s*(\d+)

#$1

#1 NO. #543

Replace some string literals

abc=1234=5678

(\w+)=(\d+)=(\d+)

$3=$1=$2

5678=abc=1234

Change the order of the groups.